You seem to not want to lose either. I'm a software developer myself who specializes in websites. If Microsoft knows a severe exploit, they probably wouldn't go around telling everybody exactly how to exploit it, would they? And we don't know that it works perfectly, just that it works enough to use it.

But we don't know if Microsoft can fix it, as it's most likely on Firefox's end.

And maybe Microsoft requires it. Also the could be more under the surface we don't know about with the user agent, where it might have some kind of security exploit or something.

No it seriously means the feature isn't available yet in the browser. Like there is a part of Firefox missing that they need to use the website. Basically all websites are coded in HTML, css, and js or a form of that. The browser controls them and the code operates out of it. If a feature is on chrome and chromium but not Firefox, the site won't work on Firefox. Not sure exactly what is missing but it is mozillas fault not Microsoft.