Seems like Flash stuff in Liferea was working fine in Fedora 8, but in F9 a worthless popup appears, asking to download a plugin but doing nothing. I already had the evil Flash plugin installed, but Liferea didn’t know about it. Maybe I did this same procedure a few months ago and didn’t remember to do it this time. Here’s what fixed it for me (based on the instructions found here):

# ln -s /usr/lib/flash-plugin/libflashplayer.so /usr/lib/xulrunner-1.9pre/plugins/libflashplayer.so

YMMV, depending on what version(s) you’ve got installed. I’d imagine it’ll be necessary to do this again if/when the xulrunner version changes.

Update: This works great for me in i386-land, but not in x86_64. I’m not sure, but it looks like x86_64 liferea isn’t using xulrunner, and I haven’t figured out if gtkhtml supports plugins at all.