I attempted to follow sbw07's instructions for applying John Doppke's preview fix to the Better GReader extension, but editing the .jar file directly in a text editor did not work for me (it broke the Firefox chrome).

Here is what did work:

I extracted the .jar archive by following the instructions here:

http://home.att.net/~cherokee67/jarfile.html

After the files are extracted, the .js file to edit is:

content\user_scripts\previewenhanced.js

Make the change indicated by John Doppke (in my version of the file it is on line 182), save the file, repack the archive, and you should be all set.

HTH