Yes, there is currently no automatic way to add feeds.
To find feeds that would be listed in the Firefox URL bar, you can look at the source code(View menu > Source Code, or ctrl-U) for a tag like
<link rel="alternate" title="Slashdot RSS" href="
http://rss.slashdot.org/Slashdot/slashdot" type="application/rss+xml">
Any feed will be a link tag with rel="alternate".