There's not really any easy way to do this. Many add-ons have scripts that they need to run in the background, so many are technically "being used" as soon as you open Firefox, whether or not you interact with them.

The only way you could really do this is if the add-on opens a specific page when you use it. In that case, you may be able to look through your history (depending on your settings) and find out that way.

Thanks for the reply‼, I kinda figured it was like what you said. I searched all my 'sqlite' db files in FF and didn't find anything obvious in that respect.

The closest thing that you would get is the extensions.json file in your profile folder. However, that will only tell you the installation date and the last update date (which is useless since updates are usually automatic).