I was getting this at home, even before updating to the latest version Firefox. After about a couple of hours or so of use, I noticed that the default font for any site I go to changes to Times Roman, and not the Helvetica that I have set to default. The other day it changed to Helvetica Ultra Light. Even changing the fonts in preferences doesn't seem to remedy the issue. The only way things would go back to normal is to quit Firefox and relaunch it. Then it would do it all over again. Sometimes it would be the next day, but it has been happening frequently. I haven't installed any new extensions or plugins just before it started. I did install Suitcase Fusion 4, and thought that may have been the culprit. But today, after updating my desktop Firefox at work, a few hours later, it too started to change it's default font to Times Roman, no Font utility installed on my system at work. Could it have to do with the last to Firefox updates. I can't seem to think of any other reason, since it all started happening after those updates.