It’s not old, the latest release 2.53.1 was released on February 28th 2020. It looks old because it was based on the old Mozilla suite, and later switched to the Firefox codebase and then to the Thunderbird codebase.
No. I mean the project they’re reviving is old.
why the fuck would a web browser use the codebase of an email client
It is a suite, and Thunderbird’s engine is the same as in Firefox but more stable.