While using Firefox, you may have experienced sudden font changes when you pressed a certain key combination by mistake, but you don’t remember what you had actually pressed. Well, if the font size changes accidentally then it simply means that you have zoomed the page in or out.
Not only the fonts but also other objects appear out of proportion. Well, in such a case you can do the following to correct it.
Press Ctrl + 0 [It’s ‘zero’, not the alphabet ‘O’]
This will restore the page size and the fonts to default.