I am giving two stars and not one because the idea is actually superb - it if worked. I have installed the add-on, re-started firefox and yet I do neither get a blue line nor a blue writing when opening facebook. And unfortunately I do not understand why. Having a look at my preferences, it says that the Facebook Container is activated and manages all tabs. Even deleting the add-on and going through the whole process once or twice more did not change anything.
I do not use any other container add-on - only if there is something pre-installed that I do not know of.

According to the comments listed herein, the add-on seems to work for most users. But I also figured out that I am not the only one having problems - which makes me optimistic that there might be a bug just needing a fix (:

Who can help?