If browser.startup.page is set to 3 (“When firefox starts” -> “Show my windows and tabs from last time”) firefox should always restore automatically the browsing session (but after a crash).
Each time after logging out (at least from kde) the restore dialog appears (“Restore session” / “Start new session”).
This is quite annoying for me since I never pressed “Start new session”, but by mistake.

The ideal solution is that logging out will not result in an improper shutdown of firefox, but I wrote a small patch that introduces the browser.sessionstore.resume_session_always setting which works for me.

I also updated Gentoo’s ebuild for firefox 2.0.0.6 to use this patch (just extract it to your /usr/local/portage directory).

Update: The above bug is fixed in firefox3. As a work around, you can create a new string preference “browser.sessionstore.restore_prompt_uri” and set it to “javascript:window.close();” (without the quotes).
Unfortunately I could not find a way to make this work with NoScript (yet).

Update #2:
Good news (:
Giorgio Maone kindly updated the NoScript plugin to allow the trick above. You currently have to use the development version (1.1.6.18).