Me too, on both my home PC and my office PC. So I don't think it's the computer. When I switch to Chrome (don't gasp, Mozilla) I have no issues.

For me too the same problem has happened since the Mozilla upgrade on two different computers, both running Win7 64-bit. IE doesn't have the same problem. And it's not just displaying pdf's. The same thing occurs on those computers when I log into a second computer via LogMeIn. In the LogMeIn window the remote desktop appears distorted and semi-transparent and my own desktop shows through as through a irregular pane of glass.

SOLUTION: see same problem posted here: https://support.mozilla.com/en-US/questions/866570 The fix they suggest (they link to this: http://blog.mozilla.com/addons/2011/08/22/firefox-6-breaking-themes-plugin/) is to go your theme publisher and have them update their theme. Here's a quicker solution: for Win 7 disable transparency under "personalization" "window color."