For Firefox, you can increase and decrease the image (zoom in/out) with "CTRL +" ("CTRL =" also works for me) and "CTRL -" (both from the numeric row and the numpad). You can reset the size using "CTRL 0". My guess is that you accidentally pressed the zoom keys while cleaning (not sure if your mouse has keys set to mimic the control keys, but if it does that could have been it, too).
"Everyone's got one. Better to be a smart one than a dumb one." - me
"It's people like you that [I]think[/I] you know everything that give those of us that [I]do[/I] a bad name." - me
"Eat, drink, and be Merry, 'cause Pippin was a fool of a Took."