Even though IE is our default browser we want users to open links from email notifications in Firefox.
So I tried:
<a href="file:///C:\Program Files (x86)\Mozilla Firefox\firefox.exe">Click me!</a>
<a href="file:///C:\Program Files (x86)\Mozilla Firefox\firefox.exe google.com">Click me!</a>
From a URL, you can't target a specific browser. That's a client preference and not something that you can specify in a URI.
file:// scheme is simply to allow you to open local resources in browsers, and cannot execute applications. Picture clicking on:
<a href="file:///C:/Windows/System32/command.com+%2Fc+"format+C:+/Q"">Click me, I'm cool!</a>
If you want that kind of control, you'd have to implement (and roll out) your own schema mechanism. i.e. making
firefox://http/somesite.com/foo/bar.htm bind to using Firefox specifically.