Someone might propose that you should use "Preferences>General>Web Browser" to add it, but I have found that doing that did not affect the list of internal browsers.
Instead, it seems (from some research) that this is related to a specific Aptana feature (Aptana underlies CFBuilder). Sadly, we may be unable to add more browsers, though, because it seems (from the URL below) that the way to do it is using the menu option "help>install aptana features > other > embedded firefox browser". But if you look, we have no "help>install aptana features" (while Aptana is embedded within CFB, it's not done in a way that exposes ALL Aptana features).
More on the proposed way (if we had Aptana for real):
Or has anyone found another way?
many thank for your research; after re-reading Release Notes of CFB, it states this as a bug [Bug 80098]
"For the plug-in configuration of ColdFusion Builder, the Firefox preview tab is not available within ColdFusion Builder."
Ah, another good point to note. Thanks.