[Solved thanks to @tal. See instructions below.]

I'd like to subscribe and occasionally post to some usenet newsgroup like sci.physics or sci.physics.research. It's difficult! I tried to simply enter "sci.physics" in Thunderbird's Newsgroup reader, but apparently it doesn't work simply like that... Even subscribing to news.eternal-september.org didn't help – I think my understanding of providers and groups is very confused.

Could anyone kindly help?