Fission locks up my PC to the point where nothing works. Even keystrokes are simply ignored. Looking at the Task Manager, I find a huge number of Firefox processes running with 100% CPU usage. If I wait until CPU usage goes down, the PC starts working again. The number of processes shown is significantly more than the actual number of Firefox copies running. Note that these are individual copies because I do not use tabs.

With Fission disabled, PC lockup is no longer a problem. The number of processes shown more closely matches the actual number of Firefox copies running, and the CPU no longer gets smacked with 100% usage.