Thank you. I already set xpinstall.signatures.required to false and that doesn't seem to have helped. I can still install the add-ons, but they don't stay installed by default sometimes and the pages where I install them from all come up with the message "Another program on your computer would like to modify Firefox with the following for each of my add-ons". Can I solve this by installing an ESR version of Firefox, and, if I can do that, then how can I transfer my old data and settings over?