Building upon Crash Recovery, this extension allows you to save the
current state of Firefox (history, text data, cookies) and return
to that state at any later moment. Besides the manually saved states,
Session Manager automatically stores the current state in case of a
crash.

All sessions are stored in the "sessions" folder inside your profile
directory and can be moved around as any other file. To get to that
folder, simply select "Open Session Folder" in Session Manager's menu
(might not work on all OSes). Finally, Session Manager also allows to
reopen the 10 last closed windows and tabs.

It is not recommended to use Session Manager at the same time as Crash
Recovery (which is completely integrated), SessionSaver or Tab Mix Plus
(which provide similar functionality on their own). In comparison,
Session Manager currently stores more session data than Tab Mix Plus
while not getting as complex as SessionSaver.

WWW: http://sessionmanager.mozdev.org/