Since Firefox 2 or so, Firefox has had the ability to save the session state (complete with tabs, cookies, and partially-completed forms). This is easily enabled through the preferences.

However, I wanted the ability to save my session on demand. It turns out Firefox has a setting for this too, which just saves the session state for the current session (one restart, then revert to default behavior).

I wanted to enable this functionality through the interface. I looked for an add-on. I looked for a bookmarklet. I couldn't find what I was looking for. I finally found this config item for an add-on which led me to my final solution. I was able to use their code to create a bookmarklet to remember the session for the next restart. Now if I'm working on anything important that I don't want to lose (by accidentally closing my browser or restarting my system), I hit the bookmark which I call Remember State for One Restart. Bah- LJ filters the link. The HREF should contain "javascript:Components.classes[%22@mozilla.org/preferences-service;1%22].getService(Components.interfaces.nsIPrefBranch).setBoolPref(%22browser.sessionstore.resume_session_once%22,true);". If you don't know what to make of that, just get the add-on below.

Since I wrote the bookmarklet, I also found Save Session, which appears to do the same in an Add-On. Good luck and happy session saving.