Sounds like Firefox may be using your old profile that has those tabs open, or there is a malware infection.

I would check for malware first: https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware#w_how-do-i-get-rid-of-malware_2. Using multiple scanners is a good idea to increase the likelihood of detection and avoid malware that can mask itself from certain scanners.

You can test for profile problems by creating a new profile when you first launch Firefox and using that profile instead of the old one: https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles#w_starting-the-profile-manager. In case this is the problem, I'll see if I can find out how to start off a new browsing session and delete the old one with the insane tab count.

You can check for problems with the sessionstore.js file in the profile folder that stores session data. When sessionstore.js is deleted you lose pinned tabs and Tab Groups and open tabs and you will have to recreate them, so if necessary make a note or bookmark them.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

If you use Sync then best is to disconnect Sync while performing maintenance/repair efforts to files in the Firefox profile folder.

According to the knowledge base, http://kb.mozillazine.org/Sessionstore.js, deleting the file mentioned in that article should erase your previous session and thus prevent previously opened tabs from loading. Again, this is only the case for old tabs, as malware may simply open new ones.

Whoops, double-timing with cor-el. ;) Please note his instructions for Sync.

PS. If you want to try salvaging from the session afterwards, you don't need to completely delete the sessionstore.js file - just move it out of the folder so Firefox won't use it.

I was told of a set of circumstances that can also lead to similar behaviour with empty tabs: https://support.mozilla.org/en-US/kb/firefox-repeatedly-opens-empty-tabs-or-windows. Sound familiar?