Probably you installed some new extensions (toolbars or something) that caused a problem making Firefox freeze. But, Firefox has a safe mode in which extensions are not loaded. To do this follow these steps:
1. Click start button
2. All programs
3. Mozilla Firefox
4. Mozilla Firefox (safe mode)
This way if your Firefox won't start at all you can use it in safe mode and disable/uninstall extensions that caused the problem.

If that doesn't work you will have to reinstall Firefox, but not before you save your precious Bookmarks, therefore follow these steps:
1. Click the Bookmarks menu.
2. Manage Bookmarks.
3. In the new window click File and then Export.
4. Select the name for your bookmarks and you are done.

Ok now that you are backed-up, uninstall Firefox from Start/Control Panel/Add or Remove Programs and then install it again.
Now import your Bookmarks from the same menu your exported them and you are now problems free.

Additional Reading:
How To Reduce Firefox Memory Usage
How to create a new profile
How to speed up firefox