This happens in Firefox 126.96.36.199 only. [EDIT: It's working fine in Safari and Seamonkey both, so it's just this version of Firefox] It's not the "new" gmail UI, as I haven't been switched to it yet. I've tried the workaround for that already, and it doesn't work. I've disabled all extensions, and I'm still getting nowhere.
I suppose I can use Safari for now, but it's bloody annoying. Anyone have any ideas what's going on?