All authors of third-party software based on Mozilla and using the extension manager hit the same problem: a lot, really a lot, of users try to install the add-on inside Firefox instead of the application the add-on is made for... I hit that issue in the past with Nvu, I hit it now with BlueGriffon, Instantbird people hit it too. The error still sits between the chair and the keyboard, but maybe we could fix it in this case: the Extension Manager could have a list of couples (appname, appid) of well-known Mozilla-based applications and report "Ooops, this is a not an add-on for Firefox but an add-on for <appname>. Please launch <appname> to install it". That would help really a lot...