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 menu button/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 also delete the addons.json file.

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

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

The files you refer to are in the Old FF Data folder generated when I refreshed, but although I can find an "extensions" folder. Search results show no extensions files any where in the Moz FF directory. Do I need to uninstall FF reinstall the whole thing?

Please excuse me I'm not very good at this. However I did follow your instruction and have located those files. Will try your suggestion to delete them.....This does not seem to have fixed anything. But also now I notice in the extensions section of the Add-ons manager that Avast security, MS.net framework assistant, default mngr, and search helper extension are all disabled.