Originally Posted by jtlaw
...never quite understood how the system "decides" to use one version or another when something like plain old "firefox" is typed at the command line...
It depends mostly on the order that locations of binaries are listed in your 'path'. To find out what your shell currently searches through, type (assuming a Bash-type shell) echo $PATH
into a terminal. You will get something back along the lines of "/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/usr/games" -- indicating that binaries (etc.) will be searched for first in /usr/local/bin, followed by /usr/bin, followed by /bin, and so on. If you type whereis firefox
into a shell, you should be able to place this into the above search path.
Unless you truly need both versions of firefox, the easiest thing to do would be to remove the old installation -- though it is also possible (likely ?) that there is a link in /usr/local/bin/firefox pointing to the old version, which you may want to delete or re-point to the new version.
Hope this helps,