I tried merging with firefox settings, but somewhy it doesn't work. Also I have tried using maximus, but that didn't work for me either.
Hopefully this will fix it overtime with version 59 of Firefox. https://www.omgubuntu.co.uk/2017/11/firefox-nightly-adds-csd-option
Update: In the latest update of Firefox (Quantum 60.0), this is now working! Just do this
- Open Menu
- Untick Title Bar
The people behind Firefox seem to be working hard on client-side decoration, which will enable this feature without any extensions. Almost every day people are working out smaller and smaller issues. You could follow the following bug to keep up to date: