Not sure why, but Firefox definitely seems to have a weak spot around 2d (SVG?) performance
I hope to do a lot more. I too, like you, am a Mozilla fanboy.
Unfortunately, in the last few months, I have a lot of pressure at my day-job (a few sudden departures). Nevertheless, I did contribute when I could spare the time and I urge you to do likewise. If there are enough of us, then there's always someone helping them out. It's a massive project and needs all the hands that kind developers are willing to spare.
At least on the versions I've tested...
I have an app that places a few hundred text labels on a canvas, and on Chrome I can do that smoothly in a single callback. For firefox I had to use a hack with a queue that is processed a few items at a time with setTimeout() to prevent it from stalling the UI for many seconds... Even so, the experience is still worse on Firefox.