This Firefox is already running bug is really annoying. I love Firefox but it's some poor UX when i manually have to shutdown the process.
I'm one of the devs who fixed it, btw.
I am not saying it's not a real issue but why it's more a problem for some then others...
(Personally, I run the Aurora channel - v31 right now on my machine - and I never have any problems.)
EDIT: Here's the relevant bug. https://bugzilla.mozilla.org/show_bug.cgi?id=1006478 If that isn't your issue, definitely file a bug report!