Rated 3 out of 5 stars

I can confirm it works after applying the patch provided at http://www.mozdev.org/pipermail/rsseditor/2009-February/000031.html

To those who asked: to apply the patch, download the RSS Editor xpi, change its extension to .zip and extract it. Enter the chrome folder and rename the rsseditor.jar file to rsseditor.zip, and then extract it (make sure its contents end up in the chrome folder itself, not an rsseditor subfolder). Now go back to the first folder you extract and (in the terminal) run

patch -p1 < /path/to/patchfile.patch

Now zip up the contents of the chrome folder and rename the newly created archive to rsseditor.jar, then delete any remaining files. Now zip back up all the package contents and rename the archive so that its extension is .xpi again.

Now, to install it, just drag your patched xpi to the Add-ons window. You’re done.

3 stars out of 5 for not being officially compatible with the latest Firefox version as of yet.