I just un-installed totem-xine and all the libraries and plugins associated with it because I'm trying out different media players.

Anyway, I installed smplayer through my package manager along with the mozilla-mplayer plugin. Now I'm trying to set smplayer as the default player for most video files in firefox (Edit>Preference>Applications) but, every time I choose smplayer.desktop (what I assume is the file I need to choose, since there is no other smplayer file that seems right and when I click on it through the file browser it launches smplayer) it just stays on "Movie Player (Default)". In fact, no applications stick when I choose them. How can I fix this?

In fact, I went back to totem-xine just to see if the problem was with smplayer, and I couldn't get some of the files to default to totem-xine either. It's weird because now all of the files are like totem-xine.desktop where, before, it was just totem-xine. I can save the video to my desktop and choose to open it with totem-xine and it works, I just can't stream it.

The .desktop files aren't application launchers, except for things like file browsers - if you actually load one of them in a text editor you'll see what I mean.

You would need to navigate to the directory where the smplayer binary is installed, usually in /usr/bin. So to do this you would go to Edit -> Preferences -> Applications, click on the action item for the type you want to configure, then choose "Use other". When the "Select helper application" window opens, you can just type "/usr/bin/smplayer" in the location bar or go to File System -> usr -> bin on the left and then select smplayer on the right. Either way you might have to wait a bit for the directory to load because this file select dialog likes to check every single file listed.