They are located in your profile directory under extensions.
Finding them is more challenging for most, since most don’t have named IDs. I am willing to bet this one does though.
For addons that don’t, you can find the ID by going to the addons page, hitting “debug addons”, then looking for your addon on that page where it will list the “Extension ID”, which should be the name it’s saved under.
AFAIK, you can find all installed XPI files in $MOZCONFIG/firefox/<profile>/extensions/You can find and access your $MOZCONFIG/firefox/<profile>, via about:profiles
PS: The XPIs dont contain the configurations of the individual extensions.