Firefox lets you choose between opening as it was last time, or as listed in your home page settings. The homepage tabs are in order and separated by a vertical | pipe symbol.

• see how to set the home page

;-) my telepathic powers are not too good today,
maybe the error messages relate to website problems. Say what they are and where and when seen if possible a screenshot would also help.

• How do I create a screenshot of my problem?
  (at present I think the forum only allows attachments on comments, but not on the original posts).

That may depend on in which window you are when you close Firefox via File > Exit Firefox.
If you are not in the main (first) window then the order might change to open the active window as the first the next time.

Hi John99 & cor-el Thanks for your replies

There is no error message displayed at any time. I think it could be as cor-el said if I'm not on the main page when closing then it re-opens as it likes !!

I'm going to look in to the setting up of home pages, as/when time allows etc... But for now thanks again...