Of course there's the Flash plugin for Firefox, but if you're running a 64-bit machine or just want to watch the movies later, here's an easy way to do it.
- Install the Ook? Video Ook! extension for Firefox. That puts a little icon in Firefox's status bar that you can click to save YouTube files locally.
- Install Nautilus Actions (nautilus-actions) and import this action (Convert FLV to MPG). An option should be added to your context menu to convert .flv files to .mpg, using ffmpeg.
Update: Instead of doing the conversion to MPG, you could also just play the FLV files in VLC.