I know this problem is limited to one browser (In this case, a fully up-to-date Firefox), but I still don’t know how to fix it. Somehow, I have managed to make the buttons that actually do things to messages disappear. I also can’t hide or show the preview pane. The light gray field where those controls should appear is super thin and completely empty.
Using another browser works for now, but I’d really rather not do this long-term.
Thanks for all assistance!