I was about ready to murder somebody the second time I ran into that, but some googling gave me the reason: it happens (on GNU/Linux) when you have updated firefox, but you are running the old program. Since opening a new tab opens a new process, this effectively means the old system would have to work intimately with a newer version -- which is too hard.

The solution is the same as always: don't upgrade until you are ready to reboot the system anyways. Or don't upgrade at all, if you can get away with it.