If you're like me and primarily used this add-on to remove the tabs from the top, follow these instructions provided by user Soulyaris:

"Inside your profile folder(it usual c:\Users\*user*\AppData\Roaming\Mozilla\Firefox\Profiles\*something*-default (check date of modified), create a new folder named chrome. Inside the chrome folder, create two new files, userChrome.css and userContent.css. At the top of userChrome.css, add the following code:
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
/* Hides tab bar*/
#TabsToolbar { visibility: collapse; }
Save it and restart firefox. Enjoy."

Really great add-on. I'm sad to see one of its core features get changed. I hope it gets fixed.