I wouldn't bother with the alias stuff. For the symlink, as root
ln -s /usr/local/share/applications/firefox-3.0rc1/firefox /usr/bin/
That removes the original firefox symlink and creates a new one in its place pointing to your new install.
The ln command is better understood once you grasp that the first path is to the binary you want to link to and that the second path is where you want the symlink to reside. If you only specify a folder (i.e. /usr/bin) then the name of the symlink automatically takes the name of the target file, or you can specify the same or different name for the symlink in the second path argument. Make sense now?
Also, any menu items that call up FF also would need to be edited to point to the new install. And you'll need to create new symlinks to your FF plugin binaries in the /usr/local/share/applications/firefox-3.0rc1/plugins directory (check to see if that is the correct path) or just copy the ones from the previous install of FF using the '-P' option
cp -P /path/to/old/FF/