I had a simillar problem. After a few hours of research and removing and reinstalling, I went and removed all installed extensions and themes from my profile.
To do this you need to "see" hidden files, to see hidden files double click on my computer, select TOOLS, FOLDER OPTIONS, select the VIEW tab, scroll down, put a tick mark on "Show hidden files and folders", click apply, click OK.
Next to get to your mozilla profile goto MY COMPUTER, double-click your hard disk (usually C), double click Documents and Settings, Select your user name, double click Application Data, Find Mozilla folder double click it, double click Firefox, double click Profiles, double click on your profile(usually theres just one folder here unless you have multiple profiles using firefox), Drag the extensions folder to your desktop or somewhere else safe. Click the back button until you are back to the Mozilla folder, Right click and delete it.
Next remove firefox completly from your machine, Use Add/Remove programs, and also goto Program files and delete the firefox or mozilla folder.
Reboot your machine, use Opera(I always reccommended having a second browser aside from firefox to avoid using IE if at all possible.) (or IE if you have to) to go and redownload Firefox. It should work this time, If it does you can go back through the above steps and One by One reinstall your extensons.