I often find myself sorting through an egregious amount of windows and tabs which I like to do based on subject. To this end, it would be efficient if it were possible to merge windows without needing each individual tab opened to do so i.e. dragging Window A into Window B causes all tabs in Window A to line up into Window B. I genuinely cannot tell you how many times I've wished for this feature, leading me to finally seek a way to formally request it.
The way it works now causes excessive memory usage, as each tab needs to be opened/loaded in order to move it. Some users have hundreds of tabs which, during the sorting process, requires multiple restarts of Firefox in order to keep memory usage in check, and a restart is necessary again once finished in order to de-load all the tabs to reset the memory usage before resuming regular usage of Firefox. This whole process is also incredibly time consuming.
Additional note, Firefox View -> Open Tabs doesn't work, showing nothing but a blank page when you have about ten to fifteen or more windows so I can't check to see if this option would be available from that menu.