What happens when you run the .exe directly?
C:\Program Files (x86)\Mozilla Firefox\firefox.exe (or just \Program Files\ if using x32)
Does it run if you right-click icon > properties > compatability 'run this program as an Administrator'
Will it run in Firefox Safemode?
"C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -safe-mode
Did you install any add-ons before it stopped working?
Did you install any other updates, like drivers
Depending on the cause, uninstalling Firefox won't help as it leaves all the data behind in:
(The Appdata folder is a hidden folder by default)
To completely remove and re-install Firefox freshly, you need to uninstall the app first and then delete those folders.
(*NB you will lose all your bookmarks, settings etc if those folders are deleted - make a copy of those folders if the issue is not with FF itself. To restore, simply copy+paste them back into place)