Whenever I update my system it seems inevitable that Firefox insists on saving my tabs, regardless of the settings I choose in Preferences.

By now I am used to going to About:Config and changing the various browser settings to "False". I confess to being puzzled that this bug never seems to go away. but it seems a small price to pay for a program like Firefox.

However on my latest upgrade—to Linux Mint 16—that solution does not work.

The most recent posts I have found on the subject either talk about deleting a user.js file—see:
https://support.mozilla.org/en-US/questions/962971

or say that one can enter that file with a text editor and change the settings shown there.

I haven't tried deleting a file yet (it seems a bit drastic).
When I opened the file with the text editor it opened "Read-Only". Reading it, it certainly shows the browser settings (that I have changed to "False" in About:Config") as "True".

Aside from this problem simply being so much more stubborn than in the past, I find it interesting that on start-up, it is always my original tabs it restores, not the ones from the latest session. I don't think it was doing that before—but then, in past years I usually had the problem dealt with within minutes.

I will be grateful for any suggestions.

There is an option in the tools menu to Save Private Browsing. That saves the current tabs for viewing later. If you knowingly, or unknowingly, created such a session, that might be the cause of the problem.

Deleting the file is seldom necessary. Just rename it (adding one character is enough). If that doesn't cure the problem, rename the file to its original name.

I am running Firefox 27.0.1. Mozilla Firefox for Linux Mint; mint-1.0.
I see nothing in Tools (or Edit>Preferences) about Save Private Browsing.
I have marked "Never Remember History" in Edit>Preferences>Privacy.

I went and had another look at lib/firefox/browser/defaults/preferences/vendor-firefox.js but it would not allow itself to be renamed (I presume because it is read-only).

You may trying to open the file with administrator (sudo) privileges. That may allow you to write to the file and change settings.

Regards

This problem seems to be solved—although it was easier than I had expected, which always makes me suspicious.
I went Firefox>Help>Troubleshooting Information and did a reset, with the intention of starting over, and replacing my add-ons after (Adblocker, Ghostery, DownloadHelper).

I cleared recent history, then set Preferences to "Never Remember History" and then closed Firefox. Very much to my surprise, it did not ask if I wanted to save tabs, and it did not save tabs. I replaced my add-ons, and Firefox continues to be on good behaviour.

An unexpected bonus was finding that the Multi-column bookmarks add-on seems to be compatible with this version of Firefox. My cup runneth over.