1. Read https://support.mozilla.org/en-US/kb/change-fonts-and-colors-websites-use#w_changing-font and see if it's stock.
2. Reset Firefox and go with its defaults.
I am using Windows 7 and Firefox with Yahoo Mail.
Well here I am again, sorry to raise so many questions, but this stumps me. My hands shake and I may have inadvertently changed an Icon. My Check Box, at the left of Yahoo messages has changed to something like a double-offset box that I don't see in the Icons.
I have tried to left-click on the desktop and navigate to \Personalize\ Change Desktop Icons\ Tried Computer, User's Files, etc. But I don't see the Square Box usually found in front of mail messages. If anyone knows how I can get it back, I'd really be happy to get the help. Thanks! Mike