Let me ask you a dumb question- how do you do that? I've been using the old Netscape control-n to pop up another browser when I need it.
Well with Firefox, on the bookmarks, work your way thru the folders to the entry that you would normally just click on with the left button, use the right mouse button instead and you will get a list of options, my system shows nine such....
for me...the top three say open, open in new window, open in new tab.... so I normally use one of the two latter ones, so I would keep each new thread in its own window, and cut and paste , reply activity for that thread in that window but on a new tab.
Now when I want to close out the thread and my activity, the other tabs, just click on the big red X in the upper right corner and respond to the question box that firefox throws up asking if you want to close the window and all associated tabs......pretty neat.
I have looked at other browsers and I don't recognize that they have the capability quite like this.