Having discovered that Chrome has pinnable “app” tabs, too, I’ve moved back to Chrome, for three reasons.
1) It’s WAAAAAY faster
2) Typing stuff in the address bar consistently loads Google search results for that stuff, whereas Firefox does sometimes, and other times it’ll load whatever site it deems the best match. While I’m sure that’s configurable, I’d rather not have to hunt for it.
3) The thumbnails of my most frequent sites is really quite useful. There’s a plugin to do this for Firefox, but it doesn’t actually do this – you have to manually add each site.
These are all pretty minor things, and of course I’ll still have to use Firefox while developing.