I know this is subjective, but I find it difficult navigating the history tab. It would be cool if the developers could update it into a more modern style. Maybe add a toggle so those who like the current style can of course keep it. It isn't really a big issue thou.
Other than that, the move to Firefox has been great. It has come such a long way in the past few years. It does eat a fair amount of ram, but I have enough spare. Browsers are constantly improving, so I can't see this being a thing forever.
Thank you, Mozilla team!