My guess is that if someone had been living under a rock and hadn't heard of any brand before, they would choose in this order:
3) Tie: Google Firefox and Opera
IE comes first because it is dead center and because of the negative space used in the graphic. It's also the only choice that correctly answers what may be perceived as a question. "Select your web browser(s)" could be interpreted as "Which is your browser?" Well, the window title says right there on the top with the IE icon that I'm using IE. Additionally, if no choice is made, IE remains as the default.
Safari comes next but it's not just because it's in the left most position but also because its graphic is outline beveled. It implies a default choice (especially being on the left). Of course depending on the page code, hitting return for this page, expecting a default of Safari could result in the "Select Later" button being activated which leaves IE as the default. There is prioritization here, although less than if it was vertical. Notice the scroll bar. This implies that there are other options past Opera and they prioritized from the left. This would also be a problem if for some reason the browser window was shrunk horizontally...Opera/Firefox might not show unless scrolled upon.
- Have this take place during the install and outside of a browser.
- Make sure all choices appear on screen without scrolling.
- Have the choices appear in random order (time-seeded and verified).
Also, I wouldn't mind if they removed IE from the list of options