Updated: October 3, 2012
Sometimes, rarely, you may encounter a situation where Firefox does not remember your choice when prompted to save your tabs for the next time it starts. So even if you do choose No, mark the checkbox Do not ask next time and quit, you will still get same, annoying message come the next browser session end.
In this article, I will show you several ways you can handle this bug and work around it, so that you can enjoy rather seamless browsing overall. It is important to note that I do not know the exact details of why and when this bug manifests, and I've seen it only a few times now and then, and I promise to keep exploring. However, I do know how to resolve the problem. So follow me.
You try to close your browser and you see this:
If you choose Save and Quit, it will most likely work. If you choose just Quit and mark the checkbox, it won't. Every time, the browser will ask the same stupid question.
First, we will need to consult the about:config page to verify that browser settings have correct values. Sometimes, your answers in the GUI prompts may not be reflected in the configurations in the background, which is what we want to examine.
The interesting preferences are browser.showQuitWarning and browser.warnOnQuick. These values should be set to false. You will notice that Firefox thinks showQutWarning is a user-set value of true, even though I've explicitly marked it as false. Some kind of a bug somewhere. Indeed, the browser continues to pester me.
Now we need to get past this problem. The first option is to allow the browser to remember the current session the next time it opens. This way, it will not prompt, but you may find this workaround inadequate, especially if you do not wish to have the last session preserved.
In Firefox preferences, under General, take a look at the Startup section. Toggle the When Firefox starts drop down choice from Show my home page to Show my windows and tabs from last time. You can also mark Don't load tabs until selected to ease on the initial loading.
A more elegant solution is to install the Tab Mix Plus extension, which takes over the built-in session manager and allows a far more granular control of your tabs and windows. If you install the extension, the warnings will also go away, plus you will be able to start your sessions with a blank page, your home page or anything else.
This is a silly howto, but it is important nonetheless. I do feel it's lacking a bit, as there's some stupid bug somewhere that takes a very special set of conditions to trigger and manifest, which makes me feel less comfortable with the two workarounds offered.
Still, it does show you how to manage seemingly impossible problems, how to work with Firefox preferences under the hood, how to manage the browser startup and how to install extensions. Plus, you get rid of your original issue. Not a bad guide, all in all. Well, if you have other useful pointers, I will sure be glad to hear them and add them.