I think I saw something about this on their support forum the other day and the suggestion was to try the current development version:

• forum: http://tmp.garyr.net/forum/viewforum.php?f=2
• versions: https://addons.mozilla.org/firefox/addon/tab-mix-plus/versions/

However, you might want to start a new thread with your specific symptoms to see whether you get different advice.

It was Tab Mix Plus. Neither Tab Mix or Firefox bothered to inform me that a new version of Tab Mix Plus was available. After I installed the newest version of Tab Mix Plus (0.4.1.7), the session restore of Tab Mix Plus worked.

Good news!

Normally Firefox checks for updates automatically. But depending on your update setting, they may not be installed automatically.

If you have Firefox set to let you choose when to install updates (instead of installing them automatically), when you visit the Add-ons page, you often will see an "Available Updates" item at the bottom of the left column, followed by a number.

I probably visit that page much more than the average person, so I'm not surprised if you never noticed it. Definitely worth checking for if any add-on is not working normally.

You also can trigger a manual update by clicking the "gear" button above the list of extensions and choosing Check for Updates.

On the other hand, if you have Firefox set to install updates automatically, then there is a problem.