An Emacs plug-in for a browser (Firefox?)

[Paul Michael Reilly <pmr@pajato.com>]

Steve Yegge, always an interesting read, recently opined that two of
his "essential" tools are Emacs and Firefox.  And claimed that if one
were to subsume the other life would be nearly perfect, in at least
one sense.

While there are a few very useful Emacs add-ons that allow you to use
emacsclient to edit text in Mozilla based tools (Thunderbird and
Firefox come to mind), I've always wondered why someone has not hacked
a plugin that support some sort of Emacs url.  For example,
"emacs://x.y.z/some/file/or/other.txt" when presented to the browser
would fire up a tab on the browser with a full blown Emacs instance
"embedded/buried" in it and edit the file other.txt with the full
power of Emacs immersed in the browser.  Sandbox issues aside, what
other issues need to be overcome to create such a browser plugin?

-pmr