Bugzilla is fine. It looks OK. You can subscribe to individual tickets. It doesn’t support emoji reactions or Markdown but it does the job just fine. One thing it does better than GH issues is ticket dependencies even with tree/graph visualization. GH just has no blockers/blocked issues in any form or shape. New ticket form has a few more fields than the default GH issue form but it’s much better than it used to be back in the days and comparable to custom form on GH. You also can log it with your GH account so not much of a barrier there, too. Seriously, try Mozilla’s bugzilla now and see for yourself. It’s probably not as bad as you remember it.

Also consider that Mozilla’s bugzilla contains quarter of a century worth of tickets. There’s no way it can be moved to GitHub (or wherever) without loss of data.