Try using the latest Firefox (v. 4.0).

We have a security camera w/software at work from Bluecherry ( bluecherrydvr.com ) which has 2 simultaneous streams: to a software client, and to an mjpeg web interface.

I can on the N900 start OpenVPN to get into the network at work, then start Firefox, go to the web address of the mjpeg stream, and watch the camera stream on the N900.

Firefox 4.0 is displaying the camera perfectly, but the N900's default browser MicroB just flashes frames alternated with white. It is likely something different in the way they handle mjpeg.