tldr I had some old custom userChrome lying around breaking things. What follows is my odyssey of figuring that out.
Indeed with Firefox in Safe Mode (extensions disabled), everything renders correctly.
According to Firefox’s help page on Safe Mode,
If the problem does not happen in Safe Mode, it is most likely because of an extension, theme or hardware acceleration.
So I started going down their recommended list:
-
I tried disabling all of my extensions manually.
→
Nope, still rendering wrong. -
I tried turning off hardware acceleration.
→ Nope, still rendering wrong. -
I tried switching to the default theme.
→ No difference. -
I tried turning off Firefox’s built-in Content Blocking.
→ No difference.
No cake. Then I found a Community Support post with more detail:
Firefox Safe Mode also: […]
- Disables the userChrome.css and userContent.css customization files
Oh. That sounds like something I might do! 
- Sure enough I found my beautiful scrollbar mod in
~/.mozilla/firefox/<profile_id>/chrome, and moved it elsewhere.
→
Everything works.
My scrollbars are less beautiful, but form follows function.
I feel dumb. Thanks for your time!