Support Firefox

[texodus]

This PR is a tracking issue for adding official Perspective support for Mozilla Firefox

Perspective (as of this writing) runs on Firefox already, but does not officially support it (Chrome only), principally because we do not currently run our integration test suite against this browser on our Github Actions CI. Without the test suite, there is no mechanism to assert that any fixes we do add for Firefox continue to work (or are not mistakenly overwritten in the very next PR), which makes it hard to justify accepting whack-a-mole PRs which attempt to address these issues.

As a starting point, we'll need to run our playwright test suite against Firefox on CI and see what breaks - from there, we should be able to put together a proper roadmap of the work left to do to get every test feature working.

A (not yet exhaustive) list of reports of Firefox issues follows. We'll want to add tests for in the course of this task, as I've closed them to minimize "Issues" noise. If you are reading this and about to report a Firefox issue, please post as a reply in this thread instead - I will likely close a re-direct the Issue here otherwise!

#2357
#2115
#1983