Firefox sure has a devoted user base, some of whom go through great lengths and cost to promote the open source browser. Take for instance Nathan Lineback of Toasty Tech, who made pencils with Firefox logo and give 'em out to his office mates.
Here's his account on how to make (buy) your own Firefox logo pencil: Link - Thanks mikolka!
//haven't used a pencil since grundschule.
Fans of anything will go to extreme lengths and expense to show their appreciation of something.
Firefox is free. They make their money (and lots of it) from search revenue when people using Firefox. I prefer Firefox, and as a web developer I would prefer my users to use firefox. So by spending my money promoting firefox, I am actually helping myself out as well.
Without Firefox fans, Firefox would not be successful, since Microsoft has the market so dominated with Internet Explorer.
Word of mouth is free.
Loosen up the pursestrings, and live a little for once.