Posters have used the word tabs to refer to many things:

(1) Tabs saved as your home pages

(2) Tabs saved when you closed your last session

(3) Buttons on the Bookmarks Toolbar

For #1 and #3, these definitely should be transferred over, although of course it's always a good idea to make a backup before any major upgrade.

For #2, I'm not sure. You can bookmark all the open tabs in your current window using Bookmarks > Bookmark All Tabs so that you have that set for future reference. This is different than restoring an earlier sessions because the sites may require you to log in again.

Does this help?

Tabs that are currently open and are restored automatically via session restore if you start Firefox are saved in the sessionstore.js in the Firefox Profile Folder.
If you mean that then you can make a backup copy of that file to be sure of that.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Session_Restore
• http://kb.mozillazine.org/sessionstore.js