A browser used by the user for browsing the is huge distances away from browser plugins that hijack the home page for purposes unrelated to the initiating task.
I've certainly seen Google's toolbar bundled with pretty much anything not related to searching the web. Yes, it's easier to remove, but it's damn annoying after clicking Next Next Next, that Google Toolbar is installed.
Also, you'd never find a shitty site like baby-face-maker or whatever advertising on Google.
I thought Firefox was supported through AdSense at least in part? The default home-page for a fresh install is a customized Google search. If you click on an ad after your search, Mozilla gets a cut.