Although I would first suggest looking at weather your machine needs a clean recovery as it sounds like firefox is not a problem, it is just struggling to run!
Anyway to move back to IE, uninstall firefox, then go to start/control panel/add remove programs/windows components tab/re-check IE/ click next and finish. Then update IE.
This is of course assuming that you uninstalled IE after installing firefox. You should only really ahve one browser installed at any one time.