I using firefox to work with different websites and mostly I have quite a few tabs open. On the one day I just look climbing routes up but later on I get back to look up stuff for uni and so the on. The result is that I have tabs to each topic mixed up since firefox opens new tab links at the end. With firefox for windows I just open a new window, maybe push it to another desktop. On my phone there is no way to keep them sorted.

So I would suggest the implementation of something like GROUPS for TABS like windows on a PC. That could be realized by another button at the top of the Tab screen, e.g. next to the "+" button which is opening a view with different "windows"/groups. In the Tab screen you would just see the tabs opend in this window, like on a desktop PC, and keep your tabs organized.