Hopefully this is useful to someone. To make Mozilla or Firefox pass AIM URIs to Gaim, do this:
As root, create the file /usr/bin/gaim-remote-uri with the following contents...
/usr/bin/gaim-remote uri $1
Then make the file executable by running...
chmod 755 /usr/bin/gaim-remote-uri
Open your prefs.js file (~/.firefox/default/XXXXXXXX.slt/prefs.js) and enter the follwing at the end of the file...
Save prefs.js and then open Mozilla/Firefox.
Try out an AIM URI (Try sending me an IM: jholmstadt).
Potentially, this could method be simplified by passing the "uri" argument to gaim-remote directly in prefs.js. However I don't know how to do this, or if it is even possible.
Alternately, I just found this howto on how to do basically the same thing, but in a different way: Gaim Firefox and Aim: Links