Firefox actually has the weirdest performance in some areas:
- clearing comment forms on entry pages (the whitespace between entry and form works as it should in _every_other_browser_; in Firefox the form doesn't clear by more than a few pixels unless I about triple the top margin).
- arranging cell space on comment forms under 360px width on mobile is not going very well (our comment forms are actually tables nested within tables, which doesn't make it any easier)
- displaying padding in text fields (for example, the search box in the navigation bar above the header is the wrong shape/size in _every_other_browser_ because Firefox)
- certain CSS properties/values work in Firefox but in no other browser, which led to this thing today where my comment forms looked like someone shook all my pages really hard and let the form fields land where they would. Such fun.
I think I'd need about a thousand hacks for every possible browser/OS/device configuration to work around some of this, or to simply hack Firefox to allow better display in other browsers. And I could hack Firefox, because there are hacks for it.
The other thing is how needless it is to test page display on multiple versions of modern Webkit (only speaking of Windows browsers). If you check a web page in Chromium or Google Chrome, it seems you've checked it in every modern Webkit browser including Safari for Windows - which stopped at v.5 some years ago - except modern (non-Presto) Opera, which has its own ideas about CSS.
I'm not sure if this applies to Chrome on Android, as well, but as far as desktop testing goes, yeah, it does seem that way.