This has been a long known issue with Firefox, and so far it doesn't seem like there is an answer, the error messages are:
"This application has failed to start because js3250.dll was not found. Re-installing the application may fix the problem."
"The Procedure entry point in JS_HasInstance could not be located in the dynamic link library js3250.dll"
some suggest uninstalling/reinstalling Firefox, creating a new profile, removing FF completely before reinstalling, downloading a new js3250.dll file, deleting a trojan file ipv6monl.dll, etc; some solutions work for some people, some work for others, here's what worked for me
In my case the problem only happened under a restricted (non-admin) user, I looked in the C:\Program Files\Mozilla Firefox folder and the js3250.dll file was there, so I ran the firefox.exe directly from there and it ran ok, then I replaced the shortcut I had to point to that location and it's working so far
hopefully this will help some other soul