Mozilla, developers of Firefox browser, generated a revenue of $53 million in the year 2005 mostly from "search engine relationships". [Jason Calacanis' estimates were close enough.]
Now it doesn't take rocket science to assume that most of this money flew in from Google as it is the default search engine in Firefox.
So how does Mozilla generates those millions of dollars from that 200 x 22 Google search box sitting on the top right corner of Firefox ? The answer lies in "client=firefox-a"
Type in any query in that box and hit enter [make sure Google is selected as the default search engine] - Now take a close look at the Google URL that appears in the Firefox address bar - you'll see a substring saying client=firefox-a.
That is sort of a hidden instruction from Mozilla to Google search engine saying the search originated from Firefox search box and if the user clicks on a sponsored link (Google Ads) displayed in search results, give us a commission.
There are millions of Firefox users worldwide searching Google several times a day and that keeps the cash registers at Mozilla ringing.
To clear any confusion - Google doesn't pay Mozilla when somebody downloads a copy of Firefox from Mozilla servers. It's only when users perform a search from Mozilla products and clicks on the Google ad.
Update: Mozilla gets revenue from other browsers as well including Microsoft IE and Opera which is free now. Only the reference string changes in that case.
Firefox - http://www.google.com/search?q=test&client=firefox-a
Opera - http://www.google.com/search?q=test&sourceid=opera
IE 7 - http://www.google.com/search?q=test&sourceid=ie7
Update: If you are curious to know how Firefox passes that parameter to Google, follow these steps:
» Open the Firefox installation folder [generally c:\program files\mozilla firefox]
» Open the google.xml file in notepad [located in searchplugins folder]
» You will see the firefox-a string in the file.
Related: Firefox Search Bar Hacks