I have zeroed this down to being a problem with the ABP add-on...
The window & tabs I originally had open comes up (inc. the new link that I'm wanting to open) but a completely fresh window opens (FF home page) at the same time.
As soon as I disable or remove the ABP add-on this issue goes away...
This issue doesn't manifest itself if I'm just launching FF (ie. not launching it by clicking on a link in an email or elsewhere).
How to fix?
Thank-you.
