I have not seen that error message before, but there are some past threads that have solved this error by resetting Firefox. (Instructions are below)

However you can check the profile folder to make sure that the application has permission to make changes by giving System full control of the profile folder: Profiles - Where Firefox stores your bookmarks, passwords and other user data

Instructions: The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions and some Preferences.

• Open websites will not be saved in Firefox versions lower than 25.

To Reset Firefox do the following:

For Firefox 29.0 and above:

1. Click the menu button , click help and select Troubleshooting Information.Now, a new tab containing your troubleshooting information should open.
2. At the top right corner of the page, you should see a button that says "Reset Firefox". Click on it.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

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Thank you.

This can happen if you try to install an extension that needs to be unpacked to be able to work properly.

It is possible that there is a problem with the file(s) that store the extensions registry.

Delete the extensions.* files (e.g. extensions.json, extensions.sqlite, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry.

• https://support.mozilla.org/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.org/kb/Unable+to+install+add-ons

If you see disabled or not compatible extensions in "Firefox/Tools > Add-ons > Extensions" then click the Tools button at the left end of the Search bar to check if there is a compatibility update available.

If this hasn't helped then check for problems with the prefs.

You can delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release.

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

Note that Windows hides some file extensions by default. Among them are .html and .ini and .js and .txt, so you may only see file name without file extension. You can see the real file type (file extension) in the properties of the file via the right-click context menu in Windows Explorer.