Sometimes firefox doesn't know the right program for the task, or doesn't provide the one that I want to use as an option. The common example that I've come across for this is VLC and Amarok (1.4 - installed from non-Ubuntu repository), which seem to never appear for me for media files.

The only way to pick them is by knowing to find the executable files in /usr/bin. This is a problem, as movie player doesn't always do the job. I am not sure by what method the suggested programs appear, but there should be something between using one from a list and finding the executable manually.

Note that VLC and Amarok appear in the applications menu. In case someone says that only Ubuntu-supplied applications need be considered, that is an awful attitude, as it discourages unofficial repositories, which allow for a greater range of software to be available to the user.