If you try the steps in How to fix preferences that won't save, does it help?

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

Delete the files extensions.* (e.g. extensions.sqlite, extensions.ini, extensions.cache) 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, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to check if there is a compatibility update available.

If you have security software with virtualization (sandbox) features then check the settings to make sure that it doesn't restore files in the Firefox Profile Folder.

thank-you very much for your help i tried both methods but unfortunately they did not work due to the fact that the problem it was caused by malware or a windows related problem (i noticed my network trouble shooter stopped working) anyway i ran a scan using kaspersky and it appeared to have sorted the malware problem i also used super anti-spyware i then backed up my bookmarks and reinstalled Firefox (having concluded that it may have been malware etc) and its working normally again