Firefox will launch applications with whatever program(s) the OS has installed. If there is more than one the 'Complete action using' dialog should appear. I would need to know specifics about the particular file you are trying to launch to assist you in detail.

Any .jar or .exe download causes this when you tap "Open". I get a list of applications & shortcuts (Accounts[com.motorola.blur.setup], AT&T Navigator, Books, Browser[default included - Chrome?], Calendar, Call details[contacts], Call details[dialer], ... Package Installer, etc.). Perhaps this last one points the way - am I supposed to pick a 'launcher' or installer for these lib and executable files? Still, I need to learn how to add ff to the list (it's not there for .html files even!).

An exe or jar file won't run on Android. Since there are no specific programs registered Android presents a list of almost every program installed on your phone.

For a saved html page on in the Firefox download manager I get the option to open it with any of the installed browser on the phone. Including Firefox mobile.

Both these lists are generated by Android. Firefox does little else than tell Android what type of file needs to be opened.

Aha! I'll move them to the pc. Still don't have ff listed when I try to open a .html...