Don't make browsers, make extensions. An interesting post by Robert Accettura. Rather than build a new browser based on another (like the Netscape 8 Beta), extend an existing one. With Firefox, this can be done with extensions. You essentially get a new browser anyway (by adding more than what already exists) - so why tell people to download your browser, just get Firefox then install your extension. Far easier to update (no need to build whole new browser, just update your extension if it breaks in a newer release), better use of resource, and allows you to add more and improve existing features.
I can see Google and Yahoo doing an extension that makes it far more usable with their services. Yahoo has already started with the Yahoo Toolbar for Firefox. When will Google release something? What would happen if people asked MSN to do a toolbar for Firefox? Consumers may want it, but would they provide it as Internet Explorer competes with Firefox, and it would show more that Firefox is a viable alternative?
Leave the browser to the browser companies. Let them handle the security issues and updates (contribute back if you find a bug, or can provide a patch), just work on the features you want adding. As Firefox works on multiple platforms, you have a much greater audience as well. It is far more beneficial to the consumers (they have more choice, and don't feel 'tied in').