'Workspaces' is a way of grouping related windows so that only one
group is visible at any one time. This is also know as a 'virtual desktop':
http://en.wikipedia.org/wiki/Virtual_desktop (When my computer was running Windows, I used VirtuaWin to give me
Workspaces:
http://virtuawin.sourceforge.net/ .)
(Note: you can choose to have some windows to be always visible.)
I have just one instance of Firefox, but one Fx window for each of my
workspaces, and I try to keep web-pages in the window for the
workspace related to that topic. I find it very helpful to keep
things separated in this way. (Try it - you may like it too

)
If I open a web-page directly in Fx, it's easy to do it in the right
workspace. But if I open it (remotely) from another app
(eg Thunderbird, Emacs, or a Terminal-window) it's not so easy to
control where the page opens. If it goes to a Fx window in the wrong
workspace it now is very awkward to move it to the correct one.
"Move to other window" would make it much easier.
If you didn't see how "Tab To Window" works, I suspect that you had
only one window open in your Fx. Or perhaps you didn't recognize the
function under the name they use "Join to Window" .
(I think "Move to other window" is a much better name.)
If you do have other Fx windows, and you click on "Join to Window",
in the tab context menu, you will get a list of the other Fx windows,
so you can choose which one you want to get this tab.
And thanks for mentioning "Multi tab handler". That looks great.
It doesn't do "Move to other window", but I would expect that it
would allow me to move multiple tabs when TMP can do that.
Thanks for your patience,
-LenW