Quote:Firefox 66 is the upcoming stable version of the web browser by Mozilla. The version is currently in the Beta channel and will be moved to Stable on March 19, 2019 according to the release schedule.

Firefox 66 incorporates several extensions related changes. One of them changes how extensions use local storage. Extensions in Firefox use JSON files for that currently: starting with Firefox 66, extensions will use IndexedDB instead.

Good news is that the migration happens automatically; developers don't need to change extension code and storage used by extensions already installed in pre-Firefox 66 versions will be migrated to the new storage format when the update happens as well.

Note: It is recommended that you back up Firefox before you make the change as it is not clear right now whether it will be possible to downgrade the browser after the upgrade. If there is any chance for a downgrade, back up Firefox. Mozilla does not delete the old storage files but adds .migrated to the filenames. You find them in the browser-extension-date folder of the Firefox profile.

Full reading: https://www.ghacks.net/2019/02/16/firefox-extensions-to-use-different-storage-type-in-firefox-66/