I have alot of tabs which I prefer to keep open for later use, but I keep them open in different FF windows, mostly 3 or 4. But why does FF never remember the order of the windows on the taskbar itself after closing the browser and restarting it again? Every other browser does it no problem, except for Firefox. I've asked around on different platforms and most people don't seem to have this issue. On Reddit I opened this same topic half a year ago and noone has a solution to it, actually some others have it too.
Windows 10 - b.19045.3324
For the record this is what I've done to resolve the problem:
Reinstalled FF completely clean (also scraped old folders) and installed everything else on it manually as well, which took hours to set up such as the settings and all the addons.
I then manually copy pasted all the webpages I had open from before. The closed the browser after it was all good, reopened and .. ITS MESSED UP AGAIN!
Edit: The first window is the only windows that always on the same spot, just like first guy responded. I forgot to mention this.
So... I have over 40 windows open and it's annoying that the order changes after every restart (except for the first window, that one is usually the same). I only restart a few times a week, but it is always puzzling where a window will be in the list.
Anything I knew about this back in the day I've unfortunately forgotten by now, so I don't have a solution. There is a longstanding bug on file about this with little recent action: https://bugzilla.mozilla.org/show_bug.cgi?id=1235231