One where on websites that don't explicitly set their form background to white, which is the default on most browsers, Firefox paints them the color of the user's desktop theme, even when the theme is dark. Black text on black background. This bug had been known about for almost 20 years and is marked "won't fix" because the devs still assert that universal user theming of websites is just around the corner. The bug manifests on mozilla.com
Another is that hard refresh sometimes flat out doesn't work. I'm a developer who occasionally makes changes to frontend code, and sometimes Firefox just decided that it's just going to cache everything and ignore what I tell it to do. Ctrl-refresh, shift-refresh, clear cache, all do nothing, I have to killall firefox and restart the browser to get my updated web page to show.