Otherwise, the method of downloading an older version doesn't seem to work anymore, it just gets me a copy of Messenger that has an overlay telling me I have to update to keep using it, which I can only make go away by hitting the button and then actually updating it (by installing the garbage version through Edge).
My copy of the old (non-browser-based) app does try to get me to update when it first starts after rebooting the PC, but I have the option of "not now" or something (I don't remember the wording at the moment). And it works much better than the newer app I uninstalled - it has a memory-resident piece that knows when a call is coming in even when the main app is not open, which the Edge-based app did not.
I use Firefox, which is not a Chromium browser, and Messenger audio/video calls no longer support Firefox, so I need to use a Messenger app that is independent of the browser. For all the folks who are using Chromium-based browsers, I don't know why you need an app at all, because apparently you can just do all your Messenger stuff in a normal tab of your browser. I leave a Messenger tab open at all times, but that no longer works for calling.
I was getting the "not now" for the last week or so, with the date I had to update by going out a day further with each day that passed, so I was ignoring it. Yesterday, however, it took that option away from me. I don't do calls or video, so the tab-based version (which I didn't know was a thing, that Messenger had its own site, basically) will be fine for me and my mom, but this decision Meta and Microsoft have made is obviously still bad, as with cases like yours. It's a really bizarre business move, part laziness and part throwing away some control over a product.