Firefox is indeed a bit of a hit-and-miss these days (though we absolutely want to support it). Best experience currently is on Chrome(ium).

Any ideas why it is hit and miss?

Do you test as much on FF as on Chromium based browsers, during development, as well as in quality assurance steps? I could imagine, that some devs are looking at it in Chromium (or even Chrome shudder) all day, while implementing things, and then just having to tick a checkbox for FF "Yep, seems to work.". Which would of course lead to a Chromium-centric code.