This application has failed to start because js3250.dll was not found
From MozillaZine Knowledge Base
Firefox may fail to start on Windows or it may close unexpectedly with the message, This application has failed to start because js3250.dll was not found. Re-installing the application may fix this problem. You must then reinstall Firefox before it will start again.
Note: If reinstalling Firefox does not allow it to start again and you use Threatfire AntiVirus from PC Tools, users report that uninstalling Threatfire (and, if necessary, reinstalling Firefox) resolved the issue.  
The error can be caused by malware which deletes the js3250.dll file and other Firefox program files whenever you perform certain actions, such as opening a desktop shortcut or Internet Explorer, or accessing the Control Panel, My Computer, or Windows Explorer.   In such cases, reinstalling Firefox is only a temporary solution, since the problem will resurface.
Checking for malware
Look for the file mskvtns.dll or the file ipv6monl.dll in the Windows\System32 folder and remove either file, if found. (Caution: There is a legitimate file named ipv6mon.dll.) Whether or not you find either of these two files, your system may still be compromised. You should scan your computer with an updated antivirus or spyware scanner and you should also also visit a Windows security forum, such as Safer Networking Forums, AumHa Forums SpywareInfo forums or Spyware Warrior Forums, for recommendations on malware detection and removal tools.
One password-stealing trojan that causes this issue is detected by Avira Antivir and is described here. This trojan creates the system file ipv6monl.dll (among other files and registry entries) and deletes these Firefox program files:
As a temporary workaround until you can get your system cleaned, you can try making the above four files read-only.