As I understand it, we're dropping support for Firefox 2? If yes, there are a few display: -moz-inline-blocks in MediaWiki core that should also be removed, and https://www.mediawiki.org/wiki/Compatibility#Browser_support_matrix needs an update.
@matmarex Thanks for bringing that up, I think there needs to be clarification on the browser support matrix. Have been talking to @Prtksxna about that already and will reach out to you and others in follow-ups. It's also debatable if getting rid of this kinda buggy vendor property doesn't still enable Firefox 2.0 to be a Grade C browser that "may lack certain visual features".
I would pledge for cleaning up and removing Firefox 2.0-only -moz-inline-block occurences from current codebase.
Hmm, I have no strong opinion on this, actually. But I'm pretty sure no one actually tests anything on Firefox 2, ever. I have Firefox 3.6 installed and test it occasionally, but not 2.
I think it's also kinda a nice surprise, if it still works for people who use Firefox 2.0 even if we're saying it's not. Therefore leaving it at 3.0+ works. Actually 3.6+ was such a great leap, that I'd go next for Grade C to this version and review the JS/CSS for code specifically targeting Firefox before 3.6+