I am replying to my own post. Tried following suggestions such as run firefox in safemode and set up a new profile but those were impossible since i couldn't open firefox. Live chat was closed (during hours posted to be open) . As a last resort i just reinstalled firefox. It worked and is fine. All my bookmarks remained intact. hope this helps however i still don't know waht caused the problem...

A possible cause is other (security) software that was using the file and thus prevented the updater from replacing the JS3250.DLL file with the newer, updated, version.