clear your internet cache first make sure you clear all 'cookies' as well, then review your system and ensure there are no tracker programs or malware running, clearing your cache usually gets rid of these but you should still check
Do an anti-virus scan. If you do not have anti-virus software then AVG free is a decent choice that you can download off the net for free, just google AVG free.
Overclocking your CPU shortens it's lifespan and I do not recommend it.
GO into Firefox's options ('Tools' -> 'Options' in the menu) and then go to the. applications tab and delete or disable any applications or add-ons you do not need or do not use.
Finally, defrag your hard drive. You can do this by going to 'Accessories' -> 'System Tools' in the start menu. A fragmented hard drive can slow down your computer significantly.
Hope this helps.