I like development in Firefox too. When I was doing it intensively (just ~2 years ago), I found some of the CSS I was using behaved differently between Gecko, WebKit and Blink, though (and differently again in Android's pre-chrome browser), so I reconfirmed that I do need to test some things across all browsers. By differently, I don't mean prefixed browser-specific CSS (although there is that), I mean standardish things like table row height calculations showed up significantly differently, certain border highlights, and a number of WebKit rendering bugs that Gecko did not have. Key-event based text editing and wheel events are also significantly different, even more so when you include the common mobile browsers.
Given the megabytes of JavaScript on many websites, the chances that some of it depends on some random VM internal quirk is quite high.