My settings open a new tab for all links.
Recently, the focus remains with the original tab and never changes to the
current or active tab.
When I use the back arrow, it changes the original tabs previous page if there was one.
How do I make sure that the page focus is always on the current tab ???
Thanks 
I use Tab Mix Plus for a long time and I’m not sure how to configure Firefox tabs without it. Presuming you are using the add-on, too, go Tab Mix Plus Options>Events>Tab Focus, where you will find pretty much detailed configuration on the tab focus behavior.
The problem is if you also want to switch to the newly opened tab using the link or not.
I tend not to want to do this, being on dial-up I want time for the page to load before visiting. So I use the middle mouse button to click on a link, this opens the link in a new tab but opens it in the background and I stay in the same page with the link. You may need to check the Tools, Options, Tabs and ensure the settings aren’t overriding your wishes.
In Firefox itself, take a look at Tools/Options/Tabs. If you untick that last item, theoretically focus should stay where you already are rather than shifting to the new tab. Keep in mind that many FF options (and I can’t for the life of me see a pattern) don’t take effect till you restart it.
Thanks Tech,
It was a setting in TabMix Plus. 
Thanks to all others for replying.
Well, now you’ve thanked me and I did nothing 
Well, I’ve read it but I agree with Rumpel’s post so I’ve skipped to post the same.
Sorry, the thanks belongs to Rumpel.
It was his post that put me on the right track.
Tech,
You can still hold onto the thanks for some future help… ;D ;D