You can reset the warn prefs on the about:config page via the right-click context menu.

browser.tabs.warnOnClose , see http://kb.mozillazine.org/About%3Aconfig_entries
browser.warnOnQuit , see http://kb.mozillazine.org/browser.warnOnQuit
browser.warnOnRestart , see http://kb.mozillazine.org/browser.warnOnRestart

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

Thanks. The first two were set already. I set the third, but it doesn't seem to help. Anything else I should check?

If the issue does not take place in Firefox's Safe mode, tick 'disable all add-ons', save changes, restart, re-enable them until you find the faulty one.

Thanks for telling me about Safe Mode. I wasn't aware of it. Unfortunately, the problem persists in this mode. I also manually disabled all add-ons before trying Safe Mode, and the problem was present then, as well. Guess I might have to do an uninstall / re-install and see if that works. :(

In Firefox 3 you do not get the 'Save & Quit' pop-up dialog if you choose Tools > Options > General > Startup: "When Firefox Starts": "Show my windows and tabs from last time".

If that option is selected then your pages will already be reopened the next time.

To get that pop-up dialog you have to select one of the other choices (Show my home page,Show a blank page).

Thanks! That gets me the pop-up I was missing. When I re-open it still doesn't open the tabs, though. It just goes to my home page. Is there another setting I'm missing, perhaps?