Tests: Run tests on both real Firefox ESRs

Tests: Run tests on both real Firefox ESRs

1. At the same time, there may be two supported versions of Firefox ESR. Run
   tests on both, installed locally.
2. Don't run tests on Firefox 115 on BrowserStack - it was added as there's
   an ESR version of Firefox 115, but ESR versions may be different, e.g. for
   some time ServiceWorker was disabled on ESR versions:
   https://bugzilla.mozilla.org/show_bug.cgi?id=1547023

Closes gh-5547