So it ended up that it wasn't that easy to switch our installer to use "Product Version" to determine what version of Firefox was installed, so I was asked to provide information on how to use the Windows Registry to determine the installed version of Firefox. Since that information is useful to everyone, I thought I would post it here.
The following registry path is where Mozilla Firefox info is contained:
(Note that Firefox 1.5 and below used to put information in HKEY_CURRENT_USER as well. Firefox 2 does not.)
Within this registry path, there is a key, CurrentVersion. This key indicates the installed version of Firefox as well as the locale. Don't confuse this with
HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla\CurrentVersion which is something else entirely.
If you need to locate the installation, there is a key under Mozilla Firefox with the same name as CurrentVersion. That key contains another key called Main that has a value for the install path (Install Directory).
Incidentally, in the screenshot, you might notice that other folder, Mozilla Firefox 188.8.131.52, which has subfolders called bin and extensions. That's there for posterity. A long time ago, we told people that to find the plugins directory, concatenate the product and version together and then look for the plugins key as a child of that key. If I remember correctly, that's how it was done even in the Netscape Navigator 2/3 days. You can read more detail about those keys on MDC.