https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save Note: Some software, like Adva

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

If the above did not help,

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

Type about:support in the address bar and press enter.

Under the page logo on the left side you will see Application Basics. Under this find Profile Folder. To it’s right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the files. Then rename or delete it. Restart Firefox.

Separate Issue; Your System Details shows;

Installed Plug-ins

Adobe PDF Plug-In For Firefox and Netscape "9.5.5" Adobe PDF Plug-In For Firefox and Netscape 11.0.12
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.