In addition to all of the reasons so many have mentioned, my favorite reason is this setting which allows me to easily use my preferred fonts on all websites: A

In addition to all of the reasons so many have mentioned, my favorite reason is this setting which allows me to easily use my preferred fonts on all websites: Allow pages to choose their own fonts, instead of your selections above De-selecting the box and specifying my preferred fonts is the absolute first setting I change on every Firefox installation.