After switching to a Firefox I installed myself (by downloading and installing instead of using Flatpak, which once had a version with a tiny cursor bug), I had to create the Ubuntu launcher shortcut myself.
No big deal; those are .ini files not unlike the Windows shortcuts of yore (and maybe now?). At some point, Firefox disappeared from the launcher, and I had to launch it by typing `firefox` in a terminal.
I finally decided on looking into this, and after the fourth "change something, log out, log back in" attempt, there was no progress. Then I saw there was a `NoDisplay=true` line in the .desktop file.
Sure enough, that hides the shortcut from the launcher. So, I got it back in the launcher, but I have no idea how `NoDisplay` got in there. Maybe I was copying another .desktop file that had something I wanted? It is a mystery.