The same Canvas code runs fine on Webkit and Blink, but really really slow on certain Firefox builds (dependent on OS and CPU architecture). I'm not at work and don't have the bookmarks handy, but you can search their issue tracker and see dozens of open issues along those lines, stretching back more than 5 years and still unresolved.

I'm sure Firefox works fine for most sites, but if you're one of those that it has an issue with, well... just like IE, below a certain % of usage share, we just have to make the decision to not support it.

Actually, this was a learning experience that caused one of our devs to switch from Firefox to Chrome too. We didn't realize how far behind Firefox had gotten until we had to debug the issue and read through their tracker. Mozilla just ignored the threads after a while and they kept building and building.

We haven't put up a warning banner yet but probably should. It's like IE all over again lol.