Hopefully Firefox will keep support for manifest 2 for the foreseeable future since they also have their own extension "store". In Chromes extension store new downloads of Ublock Origin have already been disabled and existing installations are slowly being disabled as they roll out the changes. I'm still on Vivaldi, but will be switching to Firefox very soon.

What's the last Chrome release that's going to support it? I'm going to pin that version for as long as possible.

Or you could use uBlock Origin Lite which has a MV3 version

But how long is manifest v2 going to survive on Firefox?

Firefox's v3 implementation still supports the necessary APIs, and there's a public commitment to keep them.