this bug is to track and unify the scope of "dragability" for tabs across various browsing scenarios. this bug represents one feature, who's depenencies should not be implemented separately for any given release.
Adding: bug 170628 combine all open browser windows (or the tabs in them) into one window as tabs
*** Bug 179859 has been marked as a duplicate of this bug. ***
A related concept is a "tab manager" (bug 159853). Some actions that are hard to express as drags, such as merging two windows (bug 206572), would make sense in a tab manager.
I very much desire this, but here it is being worked on, so i won't file a RFE/bug. Please bring this one home, thanks.