[BUG] Firefox: ASRouterNewTabHook.jsm, line 61: TypeError: this._router is null

[mxschmitt]

Reported:

• Cannot create Firefox browser - protocol error / Browser closed playwright-dotnet#1101
• [bug] "firefox" browser was not found. playwright-python#477 (comment)
• Firefox browser won`t start playwright-python#509 - with the normal JS variant

Upstream issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1690231

The issue itself causes a full defect of Firefox for some users.

[aslushnikov]

We get this error on every browser launch, yet only some clients report that Firefox fails to start... it doesn't look like this error is at fault here.

[aslushnikov]

I checked and the mentioned upstream patch doesn't change things for me.

We also think this is a dupe of #4660, so speculatively merging there.