Hmmm.
Possible reasons:
1. Both programs have become corrupted ;
2. There's a registry error affecting both programs ;
3. Another process is preventing the programs from launching ;
Possible solutions:
A. Do a full scan with your anti-virus software, to check that there's not something blocking your browsers.
B. Do a full-scan with the anti-malware component of your security suite. (If you've not got one, or as well, download, install and run the free version of Malwarebytes Anti-Malware:
http://www.malwarebyt...cts/malwarebytes_free ).
C. Try pressing Ctrl-Alt-Delete and clicking on the 'Processes tab' to see if something is hogging nearly all of the processing power. The percentages shown,under the 'CPU' heading should all be either '00' or very low figures. Only 'System Idle Process' should be showing a really high figure. If anything unusual has a very high figure, that's what's taking up all of your processing power.
D. Try a fresh download and installation of Firefox ;
E. If the problem is fairly recent, run System Restore (from Start > Programs > Accessories > System Tools) to take the registry back to a date before the problem first occurred .
There's a lot of guesswork in there but a registry error would be near to the top of my list of potential causes. So System Restore might be the solution (but you should still check for viruses and malware, just in case it was something nasty which corrupted the registry in the first place).
Chris