Brendan Eich has a helpful chart of Compensation of Highest paid executive at Mozilla vs Firefox market share over time.
https://twitter.com/BrendanEich/status/1217512049716035584/p...
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https://twitter.com/BrendanEich/status/1217512049716035584/p...
For example just look at the alert popups[0], or the non native contextual menus, or the video pop icon. The UI is full of little quirks like that. The tab bar is by far the ugliest one of all current browsers, at least on macOS.
It also misses important features such as multilingual spell checking. For people writing in multiple languages it's a real PITA. I know this functionality can be added with an extension but it slows down FF too much IMO.