I would recommend trying out various browser 3rd party plug-ins that are able to download videos from websites. Some claim to be compatible with Facebook. An example is Video Download Helper. This one will have an icon near the top of the browser toolbar.

Video Download Helper is a little tricky to use the first few times, but once you have that experience it becomes a breeze. I mention this one because it's one I have experience with and is mac os compatible. There might even be better ones available.
Note: I do not recommend the convert to other formats option unless the hosted format is not one you can work with easily.

Video downloads from Facebook can also depend upon the options set by the person who uploaded the video.