Try

• Firefox Button -> Options -> | Tabs [x] open new window in tab instead
  (Click to de -select if it already has a tick/check )
• see Tab preferences and settings

Thank you so much :) I should have seen that before..

Glad to have helped.

Its easy to miss the obvious, I often do !  :-)

It is often said things are
"always found in the last place you look",
true because then you stop looking, but there can be an awful lot of searching first.