Hi, tbh_commercial

Try Merge Windows

If it doesn't solved your problem, then feel free to ask in Reply Section. Thanks for raising your question in Mozilla Support Forum.

There is a pref to enable/disable this feature that you can check on the about:config page.

• browser.tabs.allowTabDetach = true

You should also have "Move Tab -> Move to New Window" in the Tab bar context menu.
If you want to move multiple tabs to the same window then you need to drag the tab(s) to that window and drop them on the Tab bar.

Thank you both for your prompt replies, but I don't understand either of the replies I received. (I'm not a techie, just a retired old fossil).

Cor-el, your reply is kind of cryptic and "over my head" -- could you "dumb it down" a little for me, plus provide more specifics? Where do I find about:config page? (And I do know about "Move Tab -> Move to New Window"...I use that all the time and that's not what I'm trying to do here.) Your final sentence about dragging the tabs is exactly what I AM TRYING to do, but it doesn't work! They won't drag! (That does work like a charm on my Win10 laptop, and USED to work on the W7 laptop but doesn't work there any more.)

Shashank, I will try the Merge Windows thing if changing the about:config setting doesn't work, but I really don't see why I should have to install some add-on to provide a function that has been just a normal ongoing feature of Firefox that I've been using for years.

Thanks again to both of you!

You can type "about:config" in your urlbar to reach the settings Cor-el mentioned, then you can search for the setting that might be flipped and flip it back by clicking the arrow on the far right next to it.

I'm wondering if you're trying to combine tabs from a private window to a non-private window, or are these windows all the same type?

You are dragging the tab to the Tab bar (or plus button to open a new tab) in the other window ?