R.I.P Firefox. Hello Opera!!!
And Mr akiha. Your solution
C:\Users\%USERNAME%\AppData\Roaming\Mozilla\Firefox\Profiles\[name].default\extensions
Is NOT working for Linux users!

PROBLEM SOLVED! For the time being.
https://www.ghacks.net/2015/06/19/how-to-disable-the-firefox-40-add-on-signing-requirement/
https://www.ghacks.net