Perhaps you have turned on dark mode? You can go to settings > extensions & themes > themes > switch to light theme and the context menu should be white now. According to Mozilla support, installing an older version won't fix most problem. But if you insist, you can go to https://ftp.mozilla.org/pub/firefox/releases/ and search for the version you need for download. However, you need to turn off automatic update in the settings > general > firefox update section if you are planning to stay on that specific version. For more, you can refer to https://support.mozilla.org/en-US/kb/install-older-version-firefox#w_i-still-want-to-downgrade-where-can-i-get-the-previous-version

I did not turn anything, just updated firefox. Downloaded 88.0 and its back to normal. Guess version 89 its the problem.