gosh if ur that desperate for an excuse to get a new pc just open up the case and unplug a component that should kill it ...then once u get a new pc just replug the component and hey presto you have 2 pcs
If you only have FireFox you are not very likely to find an exploit for it.
On the other hand if you still have IE 6 or 7 you might find one.
But I would rather recommend to just create a text file with a .bat extension with some code to delete important core windows files to make it stop booting. In the .bat file you can put something like :
del C:\path_of_the_windows_folder the path will vary depening on weather you are on xp or vista. Good luck .
PS: If it doesn't work try searching and deleting cmd.exe with the same technique.
PS2: It is not very lickely this will get you a new computer as one can always just reinstall windows .
Thats stupid why crash your computer for a new one? Oh and so you know just because you put a virus on it dosent mean that it cant be taken off. It can be fixed easily even with the strongest virus.