"Preferred Applications" doesn't offer applications for Browser and Mail Reader

Ubuntu-MATE 20.04.1, clean install, software up to date. I have Firefox 84.01, Evolution 3.35.4-0ubuntu1, and I added Thunderbird 68.10.0. I had Preferred Applications set to Firefox and Thunderbird. I've decided to revert to Evolution, and have it configured - but it is not an option in Preferred Applications; only Thunderbird is offered. So figuring Evolution might have gotten damaged, I re-installed it (via Synaptic). Now, neither slot has any options; both Browser and Mail Reader are blank, with no drop-down choices.
I can't find where those are configured; I can find ~/.config/mate-menu/applications/applications.list which has a menu configuration I've never set up and never seen. I can run any of the programs from Top Panel or Run Application.
If I save a page from Firefox, clicking on it get me Firefox, but the file does not have Firefox as one of the programs for Open With; if I click Add I get an animation, but it is not added to the Open With list.
Anyone know what's going on? Or where any of this is configured, because Thunderbird runs, but has no icons/launcher anywhere anymore, Evolution has those, but Preferred Applications doesn't know about it, and it's also lost Firefox now. Things are getting worse, not better.