A quick note in case anyone else runs across this issue.
Symptom: Google Reader only partially renders the view page — the top menu and the spinning ‘loading’ indicator in the middle of the page. This happened to me for the first time when I received the 184.108.40.206 Firefox update — though googling now it seems that people had this same problem back in March of 2007. Not sure if it was some change to webcards, Google Reader, the .8 update or a combination of these.
Diagnosis: After trying clearing the cache, cookie and restarting Firefox to no avail I went in and turned off all of the add-ons that I had loaded. Reader worked again. Next was the slow process of turning add-ons back on one by one until the offending unit was identified. webcards was the culprit. Note that it is only necessary to disable the webcards add-on, not to uninstall it for Reader to become responsive again.