Choosing the "Use custom settings for history" setting doesn't make any changes to history and cookie settings.
Firefox shows the "Use custom settings for history" setting as an indication that at least one of the history and cookie settings is not the default to make you aware that changes were made.
If all History settings are default then the custom settings are hidden and you see "Firefox will: (Never) Remember History".
Never Remember History" means that Private Browsing is active and "Always use private browsing mode" gets a check mark.

• http://kb.mozillazine.org/Preferences_not_saved
• https://support.mozilla.com/kb/Preferences+are+not+saved

My problem is that the reversion prevents form filling history being stored as well as the preferences for Add-ons. I am minded to export passwords, bookmarks then uninstall and re-install firefox. I also had a problem with BBC iPlayer. I couldn't view on Firefox but had no problems with other browsers.

If you see "Remember history" then you have the default and that should mean that remember form data is selected.
You can verify that via the custom settings choice.
That choice won't stay selected if all settings have the default value like I wrote above, so in such a case it is merely for inspection or to make it possible to make changes to the default settings.

If Firefox isn't storing form data despite that the remember form data setting is selected then there can be a problem with the formhistory.sqlite file that stores form data.

Rename or delete formhistory.sqlite (plus formhistory.sqlite-journal and formhistory.sqlite.corrupt, if they exist) in the Firefox Profile Folder in case there is a problem with the file that stores the "Saved Form" data.

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

That will remove all saved form data, so you may want to rename formhistory.sqlite to formhistory.sqlite.sav or move the file to another folder in case you need to recover some saved data.

If that has worked then you can remove the renamed files.

I decided to use AppZapper to get rid of Firefox after saving Passwords, Bookmarks and a screenshot of Extensions before re-installing - seems to have made the browser more stable as it now remembers preferences.