After switching Catalina to dark mode several parts of FF's UI seem to be broken. It seems like the browser has problems applying the correct colors to the elements, which most of the time results in poor contrast between background and text. I've noticed these problems on window titles, dialogs, tooltips and lists. Regarding the latter, the problem seems to be corrected as soon as the element is on focus, and reverts to the wrong colors when focus is lost.

Interim solution to at least improve readability is to refresh FF while on light mode, and then switch the OS back to dark mode. After this, change FF to dark mode. End result is not ideal as FF's chrome becomes dark but affected elements remain light.