Clippings stores its data in a database file located in your Firefox profile folder, under storage/default/moz-extension+++<identifier>. To find the value of <identifier>,
open a new browser tab and navigate to about:config, locate the
preference named "extensions.webextensions.uuids", then in the pref
value, search for "{91aa5abe-9de4-4347-b7b5-322c38dd9271}:" and look for the text in quotes immediately after the colon (:).
Once you know where the Clippings database files are located on your system, you can do the following to delete the clippings database:
1. Quit Firefox.
2.
Open the file explorer and navigate to your Firefox profile folder, then go to
the subfolders storage/default.
3. Delete the folder with the name "moz-extension+++<identifier>" (NOTE: There may be another folder with a similar name but ending in "^userContextId=<series of digits>" -- you can ignore this folder)
4. Start Firefox. The clippings database file will be automatically regenerated.