I reckon it's the only thing I use on macOS that even has a context menu. What's wrong with it? Or is it just that it doesn't look like those in... whatever you're using that also has them (Numbers.app et al. maybe?)?
Literally every other application on my Mac. I don't know of any Mac applications that don't have a useful context menu.
Even if you ignore Firefox's weird horizontal menu items, it looks wrong, and it acts wrong.
The dividers have a non-standard color. Some menu items have tooltips. One item has an icon, and it doesn't highlight correctly. When dismissed, the menu disappears sharply instead of fading out.
(To add insult to injury, the menu doesn't even use the system language. There's some internal setting that nobody can find which causes it to use Japanese even though my system and Firefox prefs are all set to English.)
The position is wrong, too. I'm used to dragging one pixel to the right, but that's not far enough to highlight the first item in Firefox. Many keyboard shortcuts are missing, e.g., the common ways to jump to the top/bottom item (home/end, cmd-up/down) don't work.