How are they going to do it?
Simple. They are giving away money.
If you have a website or a blog, all you need to do is to download their script and put it on your site. Your site will then detect a user if he is using Internet Explorer and will automatically prompt him to convert to Firefox. You will receive US$1 for every person that converts to Firefox from your site thanks to Google. Please note that you need to sign up with Google Adsense in order to receive the bounty.
Here's an excerpt from their open letter on why they are doing this:
About 8 months ago, we spent some time talking about an aggressive strategy to get people to switch to Firefox. Remember those splash pages on websites that say 'You must be using Internet Explorer to view this page'. What if it was the opposite? What if websites said: 'You cannot view this page with Internet Explorer. Please download Firefox to continue.'
It's a tough strategy. On the one hand, we knew it would convert way more people than classy-but-tiny "get firefox" buttons. But would bloggers and site administrators really put a splash page between 90% of their readers and their own content? Firefox fervor had reached a point where we thought that some people might actually do it, web-designers who do constant battle with IE6's lack of standards support. But we knew a lot of sites wouldn't. And we were really busy. So we tossed the idea onto the ol' idea pile and kept on trucking.
Then a few days ago, Google announced that they would pay $1 for each referral to Firefox with Google Toolbar. The idea popped back out of the idea pile. Google has just set the stage for Firefox to literally "take back the web" and go from 11% of browsers to over 50%. If people can now spread Firefox, stick it to Microsoft, and make money for each user switched, an aggressive strategy just got more appealing.