I've used it on Linux since 2005 at least, and my only problems were importing a big Outlook .pst and having trouble with its formatting and attachments. It's open daily on my machine, and while it has become more memory hungry, the CPU usage is minimum.

Maybe it's because I only use POP/SMTP accounts?

On my Ryzen 7 4800H (8 core) 16GB laptop running Linux Mint, Thunderbird Mail is using 290MB-360MB of RAM. Mostly no registered CPU usage, but when I clicked around a few folders, it went up to about 3% very briefly.