from https://www.mozilla.org/firefox/66.0.4/releasenotes/

A small number of add-ons may be listed as unsupported or may not appear in about:addons. Their data is not lost; users should be able to re-install the add-ons and recover the data. More support information.

I did find the solution to my problem. There was a suggestion about changing the master password. I did that, restarted Firefox and was able to install the LastPass add-on.

Try to rename extensions.json (extensionsOLD.json) and delete previously used extensions files like extensions.ini and extensions.sqlite file and rename compatibility.ini (compatibilityOLD.ini) in the Firefox profile folder with Firefox closed to reset the extensions registry.

• https://support.mozilla.org/en-US/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• https://support.mozilla.org/en-US/kb/unable-install-add-ons-extensions-or-themes

You can check if there is a update available in case of issues.

• click the cogwheel button (left of the Search bar) on the about:addons page to "Check for updates"

If this didn't help then delete add-ons.json and possibly prefs.js in case there is a problem with preferences.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:preferences page.

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

Hi rogermoses1969, since you answered your own question, you can mark your reply as the solution:

https://support.mozilla.org/questions/1258587#answer-1220176

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