I use Thunderbird for RSS, too. Once got bitten by some browser extension that stopped working all of a sudden, and didn't let me export (except copy pasting each feed URL on its own, with clicks inbetween each time).

My most important/only rule for feeds is: Add nothing that updates more than once a day. Such websites are better off as bookmark in the browser.

And from my ~200 feeds only a fraction actually hit that limit. So I have a nice manageable chunk of unreads every day!