Using the WebCT and/or Blackbord web app for my university courses (typically to access/download pdf's and power point presentations), Firefox will stop responding. Occassionally it will start working after a few moments, but often it has to be closed completely. I have only had this problem since the version 4/5/6 upgrades (all three versions), but had no issues with 3.6.

Along the sames lines, I had a problem where scroll bars would not appear, but this seems to have been fixed in v6.