Remove Firefox Quantum to use old extensions in old Firefox
If you tried and installed the latest, fastest Firefox Quantum, this could effect your old extensions like Gmail manager, secure login, tab mix plus etc.
You won't be able to use them, but you can remove Quantum and re-install the last working version of Firefox. This version is Firefox 56.0
Anybody running a version newer than this will find old Firefox Extensions no longer works.
Firefox 56 is the last version of Firefox that supports legacy add-ons. Updating the latest FF to FF56 will allow the old extensions to work again.
To get your old extension back working again, first download the installer. Simply run it and your old extensions are back. Note: If you manually removed your profile, you must have a backup of your old ff profile. Quantum will NOT delete or overwrite it during the installation, so by default it will be there, when you reinstall ff56.