1. Open your profile folder. Click Help > Troubleshooting, under "Application Basics" next to "Profile Directory", click "Open Containing Folder"
2. Close Firefox using Firefox button > Exit OR File > Exit if using the Menu Bar. DO NOT use the "X" close button in the upper right corner.
3. In the Profile folder opened in step 1 above, delete any of the following that you see (they will be rebuilt when you restart Firefox):
   • extensions.ini
   • extensions.cache
   • extensions.rdf
   • extensions.sqlite (Firefox 4 and above only)
   • extensions.sqlite-journal (if found) (Firefox 4 and above only)
4. Close the Profile directory, restart Firefox and test

See:

• https://support.mozilla.com/en-US/kb/Unexpected+installation+error+-203+when+installing+add-ons
• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions_-_Firefox#Unexpected_installation_error_-203

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I deleted these files and now I can not use Firefox anymore. It would not start.

I tried to download a new Firefox but I can not install it because I get " Firefox must be closed to proceed with the instalaltion"

But there's no running firefox , so how can I close it when I'm not even able to start it?

It's working now , both Firefox and No script. Task manager showed a Firefox running I just hit End Process

Thanks for your help