Did you install x64 FF or x86 FF? The older version if x86 probably didn't transfer the session to the newer version.

Thank you for your quick response. I don't know which version was installed. It "magically" appeared a few days ago when I loaded FF after two weeks away from my computer. I assume Mozilla was updating everyone's FF to 57,0. I was not given the option of declining or postponing.

Since Tabmix is an extension that has been discontinued, I'm guessing that FF 57 didn't load Tabmix files in the installation. The files are still sitting in their original folder but I don't know how to convert them or otherwise restore the tabs.

I'm guessing that FF 57 didn't load Tabmix files in the installation

No that would not have been done under any circumstance. https://www.howtogeek.com/333230/why-firefox-had-to-kill-your-favorite-extension/

Well it took 5yrs to make 57+, Developers had over a year to redo their coded, (a large amount of addons are abandoned and not been updated in 5-8yrs) and for 6mos or more prior to the Advertised Launch Date Nov 14th this was discussed all over the web. As well your addons were marked in a yellow box as Legacy.

Mozilla ok;s Extensions submitted by the Developers to be used with Firefox. They ok them only and give the Developer a web page to distribute their Extension from. This in no means that Firefox Support does the Developers Support when their page has a Support link and a contact Link.

fyi: https://mozilla.github.io/extension-finder/ https://addons.mozilla.org/en-US/firefox/extensions/

You can go back for a limited time to rescue data but there is nothing to put them in or convert them.

But this is already being done. This option of Customization is now done by you. https://www.howtogeek.com/333110/how-to-customize-firefox-quantum-and-remove-the-white-space-around-the-title-bar/ and https://www.userchrome.org/ https://www.userchrome.org/what-is-userchrome-css.html and https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/ Also a Firefox Reddit page/s

Please let us know if this solved your issue or if need further assistance.

I knew that the new version was coming, but I didn't know that it would happen that morning without notification. I ended up parsing the .rdf file in a text editor to find the lost links.

Next, I will customize FF57. Thanks for the links.