UPDATE: This is set via the Main options screen in Firefox 2. The first drop box asks what should happen "When Firefox starts." Set it to "Show my windows and tabs from last time." This is the same as the instructions below.
People who have upgraded to Firefox 2 have raved about the Session Restore feature that restores all open tabs and even data entered into forms when Firefox is restarted. I love this capability of SessionSaver in earlier versions of Firefox. Unfortunately, Session Restore was really written to handle crashes. However, you can make Firefox 2 always restore the previous session, even when you closed Firefox on purpose.
What Firefox's Session Restore feature protects against is accidental loss due to crashes (or other similar failures). Why software needs to be protected against crashing, rather than simply NOT CRASHING is beyond me. Frustratingly, however, the blurb on the Firefox 2 Features page says that it should do what I want:
You can even set Firefox 2 to always restore your previous session instead of loading a home page, so you’ll never lose your place again.
The problem is that they don't tell you how to do this. It took a little digging to find the Session Restore page at MozillaWiki, where they provide the configuration settings. Of course, the information on how to set the configuration is somewhere else. Here is the basic process:
enter "about:config" in the address bar to get the list of user preferences
type "browser.startup.page" in the filter
right-click to modify the value from 1 to 3 (1 is the default that opens the browser to the homepage only; 3 is the setting that tells Firefox to always restore the previous session)
And now that I have this working, I see that it is even better than SessionSaver because it restores more aspects about the session: it places me on the last-active tab, rather than strictly opening all the tabs again; and it is supposed to restore data in any forms.