I'm sure it's different for large projects, but for my own work, if I wait until the app is "finished" (or at least in a workable state) before working on cross

I'm sure it's different for large projects, but for my own work, if I wait until the app is "finished" (or at least in a workable state) before working on cross-browser compatibility, I hate my life and want to die. YMMV, but I find that working on compatibility from the start is the way to go. That said, if there indeed is a big performance bug in Firefox, that would totally be a valid reason not to support it (for now).