My PC, running Ubuntu from a live CD, is frozen. I think it's because I have too many Firefox tabs open, which is overloading my memory. Is there anyway to use Magic SysRq to only kill Firefox? Or to switch workstations or something to try and restore functionality?
No. The nearest thing you can do is use
Alt + SysRQ + K which should kill all processes in current VT (in this case it should mean firefox, all graphical programs you were running, your WM and X itself). Then you should be able to switch to another console (using
Ctrl + Alt + F[1-6] and run another instance of X from there (I don't use Ubuntu, but I'd bet there is a
startx script for that).
SysRQ enables you to do only very "violent" and massive things: shutdown immediately (
o), reboot immediately (
b), kill everything except init (
i), remount all filesystems as readonly (
u) ... You probably see that a single letter is not enough to convey the "killall -9 firefox" command.