If only it was easier to remove the default tabs from firefox so you don't have duplicate tabs. I recently had problems getting the userCSS to do its thing, trying different directories. In the end the problem however was that I tried to link it with a symbolic link which for some reason doesn't work.
I got it work too but wouldn't call it easy. My process involves going to about:config to enable some variable that has a super long name. Then find out where the profiles are saved and remember not to use the "cached" directory version which I always end up on first. Then selecting one of the cryptic profile names and creating some specific directory structure and copying or linking (but no soft linking) my config there.
A simple checkbox in the settings would be nice, or another browser extension. Or is there an easier process?