I use Firefox as my default browser, but I installed Chromium for my Google accounts to avoid cross-contaminating things. Somehow, this ended up with Chromium as my default browser; trying to set the default in Firefox didn't do anything. What eventually worked for me was
xdg-mime default firefox.desktop x-scheme-handler/https xdg-mime default firefox.desktop x-scheme-handler/http
xdg-settings set default-web-browser firefox.desktop
didn't seem to do anything; at first, it told me "xdg-settings: invalid application name", but trying again just now seems to work.
Computers - how do they even?