Find a solution for Firefox GL testing

It got the point where only 1 in 5 or so runs were passing so this was effecting all PRs.

The temporary workaround was to pin firefox to beta rather than dev in #14392.

Sadly I don't have more for info what kind of failure is occuring and this will probably come back to bite when beta gets updated in a few days/weeks.

An example of failing test run is here:
https://app.circleci.com/pipelines/github/emscripten-core/emscripten/13810/workflows/01da6e69-67bb-4d40-9ab4-d956eb417aae/jobs/432581

It ends with `skipped 'too many unresponsive tests, skipping browser launch - check your setup!'` which I think means the browser crashed or at least locked up.