You can check for problems with the sessionstore.jsonlz4 file in the Firefox profile folder that stores session data.

• delete sessionstore.jsonlz4 and sessionstore.js if present

When sessionstore.jsonlz4 is deleted you lose pinned tabs and open tabs and you will have to recreate them, so if necessary make a note or bookmark them.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

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

Found the reason – an old extension. Thank you for the help still.

MY SOLUTION WAS that for some bizarre reason or bug in firefox the HOME PAGE address storred all the tabs addresses in it so every time I opened the firefox with my default normally google.co.uk home page, it was actually opening about 10 addresses from one of the last sessions and then keep opening. It was so annoying as it propagated onto all my devices through my Firefox sync so:

The solution to your old tabs from your old firefox session keep opening could be that tab session with all the addresses got stored/saved into the home address field. So just change your default home page address in Firefox Options.

So when you have the problem, first hover with your mouse over Home button and see if there is only one address there or plenty one by one.