Which system and Firefox version? Are you using "always private browing" mode or other privacy options?

Edit: This is strange. I did not see this error on the same version. Also, the private browsing bug has be fixed in this version of Firefox.

I suggest using a new profile (about:profile).

If you want to debug this extension, please right click the "Add to Firefox" install button and save the link to xpi file. Open about:debugging#addons, check the checkbox, click "load temporary addon", and select this xpi file. Click "Debug" under the temporary loaded extension and then click "reload", to see if there is any error shown in the Console tab.