I am using Firefox 12.0. I use Hana FLV player v2.9.3 and it works great with FLV as the video buffer fills as the video plays. I am unable to replicate this behaviour with the HTML5 Mediaelement.js player (option 5), as the video has to be fully loaded into the buffer before it starts to play. Some the videos are quite long and it is unacceptable to wait quite a time before play starts. I notice on the http://mediaelementjs.com/ the video does play as the buffer fills, so I'm sure it is possible.
I have found that this topic has been raised before, but no solution was given.
Here is a typical example of the settings I use: