I'm assuming you don't have any hidden tabs.

If you arrange the windows so you can see both tab bars:

(1) In the window with the tabs you want to move over to the other window:

Multi-select the tabs by clicking the first one and Shift+clicking the last one

(2) Drag one of the multi-selected tabs to a point on the other tabs bar where you want to insert them.

Firefox should display a little insertion mark. Release the mouse to drop them there.

The first window should vanish, but if it's doesn't, close it.

(3) Repeat until all consolidated.

Success?

Thanks, not tried it yet [it's late...] but this sounds good: will try it tomorrow and acknowledge...

great. I probab;ly knew you could do this, just wasn't looking at the issue in this way...