I found a way to change the default browser for all actions in Visual Studio Express 2012 for Web. It's documented here:
By default, Visual Studio uses your default browser to test pages. To use a different browser or Page Inspector, right-click [an .aspx] page in [the] Solution Explorer and then click [...] Browse With [, which] lets you select a browser from a list, add new browsers to the list, or set one as the default browser. (The default browser setting here applies only to the Visual Studio environment and not to Windows.)
This also changes the default browser for F5'ing an MVC project, because I'm used to closing IE to stop debugging. When I let Visual Studio use my system's default browser, I first have to switch back to it from my browser to stop debugging, because closing the browser tab doesn't.
Unfortunately it still doesn't open links in comments in a browser.