> I.e. a low memory solution so you can leave it going and only open the client for actual interactions. Thunderbird is pretty bloated to leave open all day and sending and receiving shouldn't need a web browser attached.

I agree with you. Having my mail client still use ~800MB of memory when in the tray is not great (and I have very few emails, so one cannot blame the index or whatever).

Unfortunately since Thunderbird's IMAP stack was recently (a few years ago) rewritten in javascript (from C++), it's now dependant on the web stack to receive and send emails. I doubt firefox/thunderbird is engineered in a way that you could unload only the html rendering part on demand.

So I doubt we'll ever have our wish granted...