See this Microsoft support article for details, including a fix you can download.
 Firefox may fail to start if the Iolo System Mechanic application is installed.
 As a workaround, open the Firefox installation directory (usually C:\Program Files\Mozilla Firefox) and look for two "js3250.dll" files (the second one will be named "js3250moz-backup").
  If you see message, This application has failed to start because js3250was not found.Re-installing the application may fix this problem.For detailed Firefox instructions, see Standard diagnostic - Firefox#Clean reinstall.This error is caused by inconsistent file versions ("application.ini" and/or "platform.ini" and possibly others). Update all plugins, uninstall any plugins you no longer need or disable all plugins, to see if that resolves the problem.
See Issues related to plugins for plugin locations and additional troubleshooting information, including how to disable plugins if your browser will not start.you must reinstall Firefox (or other Mozilla browser) and check your system for malware.See This application has failed to start because js3250was not found for more information.This Firefox startup error can occur after performing a Windows System Restore (bug 877143) but whatever the reason, the usual solution is to do a clean reinstall by removing the Firefox program folder and then reinstalling Firefox (there's normally no need to uninstall Firefox before reinstalling it):    This can be caused by an incorrect setting in the Windows registry.If you attempt to install the Opera browser as a workaround, or if you attempt to install or remove various Microsoft applications, you will see an error similar to, Error 1606: Could Not Access Network Location.If the Firefox or Sea Monkey browser process from the previous session did not exit properly, the next time you attempt to start your Mozilla browser it may fail to start with no error messages.