I would suggest running in Safe Mode via Start > All Programs > Mozilla Firefox > Mozilla Firefox (Safe Mode).

Poorly designed or incompatible Extensions can cause problems with your browser, including make it crash, slow down page display, etc. If you encounter strange problems relating to parts of the browser no longer working, the browser not starting, windows with strange or distorted appearance, degraded performance, etc, you may be suffering from Extension or Theme trouble. Restart the browser in Safe Mode.

If this now runs trouble free, restart in normal mode, disable all extensions, use the original theme and it's trial and error hopefully when you re-install the extensions one at a time to see if there is a conflicting one.