The missing partition made it nearly unusable for me as well. Thankfully, I eventually found a solution at https://www.userchrome.org/firefox-89-styling-proton-

The missing partition made it nearly unusable for me as well. Thankfully, I eventually found a solution at https://www.userchrome.org/firefox-89-styling-proton-ui.html; https://www.userchrome.org/firefox-89-styling-proton-ui.html... since then I’ve had no problems.