Special Update:

Microsoft did an “update” to version 3.5 .net framework that silent installs a plug-in into Firefox without even saying a word. This update crashes Firefox. And you can’t use the add-on area of FF to uninstall it.

You will have to delete a dll file to fix this. MS did a sneaky install to FF when it did a 3.5 .net framework update.

1. Go to x:WindowsMicrosoft.NetFrameworkv3.5Windows Presentation
Foundation directory (replace x with your system drive letter which is
in most cases c)

2. Copy the NPWPF.dll file in that folder to a backup folder (or just flat out delete it if you are feeling bold)

3. Delete the file (if you weren’t feeling bold)

4. Try running FF but if you get an error then reboot the machine
and run CCleaner or Advance SystemCare to clean temp and fix the
registry.

Microsoft really is underhanded here considering what they didn’t like Google doing to IE in a similar fashion.