diff --git a/.eslintignore b/.eslintignore
index 70e57e2111..687648ad61 100644
--- a/.eslintignore
+++ b/.eslintignore
@@ -8,6 +8,7 @@ node6-test/*
 node6-testrunner/*
 lib/
 *.js
-src/chromium/protocol.d.ts
 src/generated/*
+src/chromium/protocol.d.ts
+src/firefox/protocol.d.ts
 src/webkit/protocol.d.ts
diff --git a/.gitignore b/.gitignore
index c2ae7f3ebc..311c1c0bb1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,8 +14,9 @@
 package-lock.json
 yarn.lock
 /node6
-/src/chromium/protocol.d.ts
 /src/generated/*
+/src/chromium/protocol.d.ts
+/src/firefox/protocol.d.ts
 /src/webkit/protocol.d.ts
 /utils/browser/playwright-web.js
 /index.d.ts
diff --git a/install.js b/install.js
index 12ec4b26d9..441b3e3443 100644
--- a/install.js
+++ b/install.js
@@ -26,7 +26,7 @@ try {
 }
 
 (async function() {
-  const {generateWebKitProtocol, generateChromeProtocol} = require('./utils/protocol-types-generator/') ;
+  const {generateWebKitProtocol, generateFirefoxProtocol, generateChromeProtocol} = require('./utils/protocol-types-generator/') ;
   try {
     const chromeRevision = await downloadBrowser('chromium', require('./chromium').createBrowserFetcher());
     await generateChromeProtocol(chromeRevision);
@@ -35,7 +35,8 @@ try {
   }
 
   try {
-    await downloadBrowser('firefox', require('./firefox').createBrowserFetcher());
+    const firefoxRevision = await downloadBrowser('firefox', require('./firefox').createBrowserFetcher());
+    await generateFirefoxProtocol(firefoxRevision);
   } catch (e) {
     console.warn(e.message);
   }
diff --git a/package.json b/package.json
index 1f32de3eb0..e617283475 100644
--- a/package.json
+++ b/package.json
@@ -58,6 +58,7 @@
     "jpeg-js": "^0.3.4",
     "minimist": "^1.2.0",
     "ncp": "^2.0.0",
+    "node-stream-zip": "^1.8.2",
     "pixelmatch": "^4.0.2",
     "pngjs": "^3.3.3",
     "text-diff": "^1.0.1",
diff --git a/src/firefox/Connection.ts b/src/firefox/Connection.ts
index 5f5122221e..6474713b90 100644
--- a/src/firefox/Connection.ts
+++ b/src/firefox/Connection.ts
@@ -19,6 +19,7 @@ import {assert} from '../helper';
 import {EventEmitter} from 'events';
 import * as debug from 'debug';
 import { ConnectionTransport } from '../ConnectionTransport';
+import { Protocol } from './protocol';
 const debugProtocol = debug('playwright:protocol');
 
 export const ConnectionEvents = {
@@ -144,6 +145,12 @@ export class JugglerSession extends EventEmitter {
   private _callbacks: Map<number, {resolve: Function, reject: Function, error: Error, method: string}>;
   private _targetType: string;
   private _sessionId: string;
+  on: <T extends keyof Protocol.Events | symbol>(event: T, listener: (payload: T extends symbol ? any : Protocol.Events[T extends keyof Protocol.Events ? T : never]) => void) => this;
+  addListener: <T extends keyof Protocol.Events | symbol>(event: T, listener: (payload: T extends symbol ? any : Protocol.Events[T extends keyof Protocol.Events ? T : never]) => void) => this;
+  off: <T extends keyof Protocol.Events | symbol>(event: T, listener: (payload: T extends symbol ? any : Protocol.Events[T extends keyof Protocol.Events ? T : never]) => void) => this;
+  removeListener: <T extends keyof Protocol.Events | symbol>(event: T, listener: (payload: T extends symbol ? any : Protocol.Events[T extends keyof Protocol.Events ? T : never]) => void) => this;
+  once: <T extends keyof Protocol.Events | symbol>(event: T, listener: (payload: T extends symbol ? any : Protocol.Events[T extends keyof Protocol.Events ? T : never]) => void) => this;
+
   constructor(connection: Connection, targetType: string, sessionId: string) {
     super();
     this._callbacks = new Map();
@@ -152,7 +159,10 @@ export class JugglerSession extends EventEmitter {
     this._sessionId = sessionId;
   }
 
-  send(method: string, params: any = {}): Promise<any> {
+  send<T extends keyof Protocol.CommandParameters>(
+    method: T,
+    params?: Protocol.CommandParameters[T]
+  ): Promise<Protocol.CommandReturnValues[T]> {
     if (!this._connection)
       return Promise.reject(new Error(`Protocol error (${method}): Session closed. Most likely the ${this._targetType} has been closed.`));
     const id = this._connection._rawSend({sessionId: this._sessionId, method, params});
diff --git a/src/firefox/ExecutionContext.ts b/src/firefox/ExecutionContext.ts
index 502964a11c..19ed5058db 100644
--- a/src/firefox/ExecutionContext.ts
+++ b/src/firefox/ExecutionContext.ts
@@ -18,6 +18,7 @@
 import {helper, debugError} from '../helper';
 import * as js from '../javascript';
 import { JugglerSession } from './Connection';
+import { Protocol } from './protocol';
 
 export class ExecutionContextDelegate implements js.ExecutionContextDelegate {
   _session: JugglerSession;
@@ -104,7 +105,7 @@ export class ExecutionContextDelegate implements js.ExecutionContextDelegate {
     checkException(payload.exceptionDetails);
     return context._createHandle(payload.result);
 
-    function rewriteError(error) {
+    function rewriteError(error) : never {
       if (error.message.includes('Failed to find execution context with id'))
         throw new Error('Execution context was destroyed, most likely because of a navigation.');
       throw error;
@@ -167,7 +168,7 @@ function checkException(exceptionDetails?: any) {
   }
 }
 
-export function deserializeValue({unserializableValue, value}) {
+export function deserializeValue({unserializableValue, value}: Protocol.RemoteObject) {
   if (unserializableValue === 'Infinity')
     return Infinity;
   if (unserializableValue === '-Infinity')
diff --git a/src/firefox/NetworkManager.ts b/src/firefox/NetworkManager.ts
index 07c00480e6..2512918e39 100644
--- a/src/firefox/NetworkManager.ts
+++ b/src/firefox/NetworkManager.ts
@@ -189,7 +189,7 @@ class InterceptableRequest {
     (this.request as any)[interceptableRequestSymbol] = this;
   }
 
-  async continue(overrides: any = {}) {
+  async continue(overrides: {url?: string, method?: string, postData?: string, headers?: {[key: string]: string}} = {}) {
     assert(!overrides.url, 'Playwright-Firefox does not support overriding URL');
     assert(!overrides.method, 'Playwright-Firefox does not support overriding method');
     assert(!overrides.postData, 'Playwright-Firefox does not support overriding postData');
diff --git a/src/firefox/Page.ts b/src/firefox/Page.ts
index fa3a07870a..95d6ee1346 100644
--- a/src/firefox/Page.ts
+++ b/src/firefox/Page.ts
@@ -118,7 +118,7 @@ export class Page extends EventEmitter {
   }
 
   async emulateMedia(options: {
-      type?: string,
+      type?: ""|"screen"|"print",
       colorScheme?: 'dark' | 'light' | 'no-preference' }) {
     assert(!options.type || input.mediaTypes.has(options.type), 'Unsupported media type: ' + options.type);
     assert(!options.colorScheme || input.mediaColorSchemes.has(options.colorScheme), 'Unsupported color scheme: ' + options.colorScheme);
diff --git a/test/jshandle.spec.js b/test/jshandle.spec.js
index 7d120336c6..f0c90055dc 100644
--- a/test/jshandle.spec.js
+++ b/test/jshandle.spec.js
@@ -97,6 +97,11 @@ module.exports.addTests = function({testRunner, expect, CHROME, FFOX, WEBKIT}) {
       else if (FFOX)
         expect(error.message).toContain('Object is not serializable');
     });
+    it('should work with tricky values', async({page, server}) => {
+      const aHandle = await page.evaluateHandle(() => ({a: 1}));
+      const json = await aHandle.jsonValue();
+      expect(json).toEqual({a: 1});
+    });
   });
 
   describe('JSHandle.getProperties', function() {
diff --git a/utils/protocol-types-generator/index.js b/utils/protocol-types-generator/index.js
index 5ab9885bc4..59c1d4fc9e 100644
--- a/utils/protocol-types-generator/index.js
+++ b/utils/protocol-types-generator/index.js
@@ -1,6 +1,8 @@
 // @ts-check
 const path = require('path');
 const fs = require('fs');
+const StreamZip = require('node-stream-zip');
+const vm = require('vm');
 
 async function generateChromeProtocol(revision) {
   const outputPath = path.join(__dirname, '..', '..', 'src', 'chromium', 'protocol.d.ts');
@@ -114,4 +116,99 @@ function typeOfProperty(property, domain) {
   return property.type;
 }
 
-module.exports = {generateChromeProtocol, generateWebKitProtocol};
\ No newline at end of file
+async function generateFirefoxProtocol(revision) {
+  const outputPath = path.join(__dirname, '..', '..', 'src', 'firefox', 'protocol.d.ts');
+  if (revision.local && fs.existsSync(outputPath))
+    return;
+  const zip = new StreamZip({file: path.join(revision.executablePath, '..', 'omni.ja'), storeEntries: true});
+  // @ts-ignore
+  await new Promise(x => zip.on('ready', x));
+  const data = zip.entryDataSync(zip.entry('chrome/juggler/content/protocol/Protocol.js'))
+
+  const ctx = vm.createContext();
+  const protocolJSCode = data.toString('utf8');
+  function inject() {
+    this.ChromeUtils = {
+      import: () => ({t})
+    }
+    const t = {};
+    t.String = {"$type": "string"};
+    t.Number = {"$type": "number"};
+    t.Boolean = {"$type": "boolean"};
+    t.Undefined = {"$type": "undefined"};
+    t.Any = {"$type": "any"};
+
+    t.Enum = function(values) {
+      return {"$type": "enum", "$values": values};
+    }
+
+    t.Nullable = function(scheme) {
+      return {...scheme, "$nullable": true};
+    }
+
+    t.Optional = function(scheme) {
+      return {...scheme, "$optional": true};
+    }
+
+    t.Array = function(scheme) {
+      return {"$type": "array", "$items": scheme};
+    }
+
+    t.Recursive = function(types, schemeName) {
+      return {"$type": "ref", "$ref": schemeName };
+    }
+  }
+  const json = vm.runInContext(`(${inject})();${protocolJSCode}; this.protocol.types = types; this.protocol;`, ctx);
+  fs.writeFileSync(outputPath, firefoxJSONToTS(json));
+  console.log(`Wrote protocol.d.ts for Firefox to ${path.relative(process.cwd(), outputPath)}`);
+}
+
+function firefoxJSONToTS(json) {
+  const domains = Object.entries(json.domains);
+  return `// This is generated from /utils/protocol-types-generator/index.js
+export module Protocol {${Object.entries(json.types).map(([typeName, type]) => `
+  export type ${typeName} = ${firefoxTypeToString(type, '  ')};`).join('')}
+${domains.map(([domainName, domain]) => `
+  export module ${domainName} {${(Object.entries(domain.events)).map(([eventName, event]) => `
+    export type ${eventName}Payload = ${firefoxTypeToString(event)}`).join('')}${(Object.entries(domain.methods)).map(([commandName, command]) => `
+    export type ${commandName}Parameters = ${firefoxTypeToString(command.params)};
+    export type ${commandName}ReturnValue = ${firefoxTypeToString(command.returns)};`).join('')}
+  }`).join('')}
+  export interface Events {${domains.map(([domainName, domain]) => Object.keys(domain.events).map(eventName => `
+    "${domainName}.${eventName}": ${domainName}.${eventName}Payload;`).join('')).join('')}
+  }
+  export interface CommandParameters {${domains.map(([domainName, domain]) => Object.keys(domain.methods).map(commandName => `
+    "${domainName}.${commandName}": ${domainName}.${commandName}Parameters;`).join('')).join('')}
+  }
+  export interface CommandReturnValues {${domains.map(([domainName, domain]) => Object.keys(domain.methods).map(commandName => `
+    "${domainName}.${commandName}": ${domainName}.${commandName}ReturnValue;`).join('')).join('')}
+  }
+}`
+
+}
+
+function firefoxTypeToString(type, indent='    ') {
+  if (!type)
+    return 'void';
+  if (!type['$type']) {
+    const properties = Object.entries(type).filter(([name]) => !name.startsWith('$'));
+    const lines = [];
+    lines.push('{');
+    for (const [propertyName, property] of properties) {
+      const nameSuffix = property['$optional'] ? '?' : '';
+      const valueSuffix = property['$nullable'] ? '|null' : ''
+      lines.push(`${indent}  ${propertyName}${nameSuffix}: ${firefoxTypeToString(property, indent + '  ')}${valueSuffix};`);
+    }
+    lines.push(`${indent}}`);
+    return lines.join('\n');
+  }
+  if (type['$type'] === 'ref')
+    return type['$ref'];
+  if (type['$type'] === 'array')
+    return firefoxTypeToString(type['$items'], indent) + '[]';
+  if (type['$type'] === 'enum')
+    return type['$values'].map(v => JSON.stringify(v)).join('|');
+  return type['$type'];
+}
+
+module.exports = {generateChromeProtocol, generateFirefoxProtocol, generateWebKitProtocol};
\ No newline at end of file