Try to reduce spurious failures on Windows

ci/azure-install-geckodriver.yml

@@ -12,3 +12,14 @@ steps:
       Write-Host "##vso[task.setvariable variable=GECKODRIVER;]$pwd\geckodriver.exe"
     displayName: "Download Geckodriver (Windows)"
     condition: eq( variables['Agent.OS'], 'Windows_NT' )
+
+  # It turns out that geckodriver.exe will fail if firefox takes too long to
+  # start, and for whatever reason the first execution of `firefox.exe` can
+  # take upwards of a mimute. It seems that subsequent executions are much
+  # faster, so have a dedicated step to run Firefox once which should I
+  # guess warm some cache somewhere so the headless tests later on all
+  # finish successfully
+  - script: |
+      "C:\Program Files\Mozilla Firefox\firefox.exe" --version
+    displayName: "Load firefox.exe into cache (presumably?)"
+    condition: eq( variables['Agent.OS'], 'Windows_NT' )