One of the best plugins for Firefox. Used with Tab Mix Plus and BarTab and you have most likely the best solution for managing your tabs across sessions.
One thing needs to be added: The option to have the plugin NOT prompt to load the session when Firefox crashes. Some crashes are not page related, rather because current version of Firefox simply crashes without reason. Just simply start Firefox and have to reload your last session without prompting, even after a crash.
This review is for a previous version of the add-on (0.6.9.3.1-signed).
Check the "Use browser's built-in crash recovery prompt" in the General Startup & Shutdown options and the built-in crash prompt in Firefox will be used. That prompt doesn't prompt you the first time Firefox crashes, it simply reloads the last session. If it crashes again it will prompt you. Session Manager still makes a backup of the crashed session, it just doesn't put up it's own prompt window.