I get the impression that this kind of thing depends rather on what specific locked-down settings choices the Exchange admins have made. I use work's Office-365

I get the impression that this kind of thing depends rather on what specific locked-down settings choices the Exchange admins have made. I use work's Office-365 mail setup (which is I assume basically "exchange in the cloud") with stock thunderbird plus the free "TbSync" and "Provider for Exchange ActiveSync" addons, for instance (and I only need those for calendar and contacts stuff). This relies on the Exchange server not having disabled IMAP and SMTP. So the userbase for this kind of connector feature is not just "people whose organisation uses Exchange" but "people whose organisation uses Exchange and whose mail admins are sufficiently anti non-Outlook clients to have turned off IMAP but not so anti non-Outlook clients as to just say that it's policy that you must use Outlook". (There was a period where stock Thunderbird couldn't handle a "Modern Authentication" Exchange setup, but as of Thunderbird 78 or so it now can.) More generally, I've found in the past that "open source connectors etc to Exchange" support tends to be or become abandonware, because generally the kind of person who writes that sort of addon/integration doesn't really like working in the sort of environment which mandates Exchange and locks it down, and so after a few years when they move on to a new job that doesn't require them to use Exchange the addon becomes abandonware because the original author no longer needs it. So it probably does need to be either Mozilla taking it on as a "we must support this" feature or somebody like Beonex making it a paid product.