Hi, have a work through this article - How to fix preferences that won't save, and it should help you to resolve your problem.

In case you use "Clear history when Firefox closes" or otherwise clear history:

• do not clear the Cookies
• do not clear the Site Preferences

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/remove-recent-browsing-search-and-download-history

Clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, passwords, and other website specific data.

Clearing cookies will remove all specified (selected) cookies including cookies with an allow exception that you would like to keep.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

There is software like Advanced SystemCare with its Surfing Protection feature that can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Thanks for the replies , in the end I decided to de-install Firefox , after exporting my bookmarks , cleaned every trace of Firefox including the app data , ran a registry cleaner and did a clean install : What joy !! . Now have a clean , fast and perfect Mozilla browser again . So somewhere along the line it seems an update threw a spanner in the works. Once again : thanks for the tips ( and for Mozilla Firefox of course ) !

That was very good work. Well Done. Please flag your last post as Solved Problem so other will know.