I had the most annoying problem in Firefox 3; Google Reader stopped working. I'd click on a blog and no items would show up, even if I started Firefox in safe mode.
Turns out I'm not the only one with this problem, but it has an easy solution. Press Ctrl-0 while on the Google Reader page and all is fixed.
What happened is you accidentally changed the zoom level of the page (via Ctrl-Scrollwheel or Ctrl-Minus or the like) and some bug in Reader's HTML and/or Firefox's rendering causes all the content to disappear. There's a Firefox bug filed, but they're pointing the finger at Google.
PS: dear Google Groups team, it is unacceptable for new messages posted to a group to not show up when I post them. I don't care if the backend datastore takes a minute to commit the data, figure out some way to make it visible immediately.
Update: a fix for the Reader bug is in the works.