A quick way to increase the responsiveness of firefox is to turn off the disk cache and turn on the memory cache. This will keep a cache of recently visited websites/images in the computers RAM instead of on the hard disk.
Go into about:config and set browser.cache.disk.enable to FALSE, then right-click and select NEW -> INTEGER and type browser.cache.memory.capacity
and set to 100000 (for 100mb).
Now your memory is being used for the cache instead of your hard disk. If you have loads of memory (more the 2GBs) then you could set the cache figure to say 200mb or higher for even better performance.
Firefox Performance