Rated 4 out of 5 stars
If you often have many windows and tabs open, Session Manager is necessary, because of the instability of Firefox. If Firefox crashes, and you don't have Session Manager, you will lose your work.
There are two areas in which Session Manager is less than perfect. When re-loading a session in which there are tabs that require a log-in, Session Manager may ask for the master password many, many times.
Second, Session Manager does not remember the window size of each window.