STR:

1. Load the attached extension.
2. Navigate to https://example.com/redir (or click on the extension button).
3. (the extension will use the webRequest API to redirect the request to a web-accessible moz-extension page).
4. Look at the page.

Expected:

• "events,types,browserAction,tabs,windows,extensionTypes,contentScripts,management,permissions,theme,experiments,webRequest,extension,i18n,runtime,test"
  (this shows that the page is loaded in the extension process)

Actual:

• "extension,i18n,runtime,test"
  (the page is loaded in a web content process, gBrowser.selectedBrowser.remoteType is "web" instead of "extension"; checking the process ID in the content toolbox also shows that the page itself is loaded in the wrong process)

This is not a recent regression. It happens on Nightly 70, release 68.0.1, Firefox 60 (tested on Linux).

When the tab is reloaded or duplicated, the page is loaded in the expected process.

We're hoping this will get fixed via fission changes (assuming it's not already fixed).

With fission.autostart=true, this bug goes away.

This issue has been fixed since Firefox 72 by bug 1584031 (https://hg.mozilla.org/mozilla-central/rev/7a1d40167829), but regresseed briefly in 73 (https://bugzilla.mozilla.org/show_bug.cgi?id=1561711#c8), so we should add a unit test.

Depends on D68608

Thanks for taking over Will!

https://hg.mozilla.org/mozilla-central/rev/8a6011666e5f