Firefox keyboard conflict: Command key, Control key, and Contextual Menus
The contextual menu problem was resolved when I updated my version of Firefox from 17 (December 2012) to 23(!) in September 2013. The built-in update button within Firefox updated my app to 17.0.2esr (security updates) and offered no info about the later versions. 

Only after searching other Firefox questions here did I notice someone mentioning version 20 and up. I do not know why Firefox 17 did not issue an alert or a chance to upgrade itself to a higher version, or if it did, why I had not noticed that as it climbed from version 17 to 23.

The noncontiguous selection problem (clicking while holding the command key) still remains, but I suspect it is a problem with PHP5.0, the language the Web app is programmed in, or a bug within the app's code.