I raised a bug over a month ago with the RC's, and so have multiple other people, it was raised to critical but the decision has been made to rush out FF3 without resolving a serious bug that affects us all who use Ip Cameras
https://bugzilla.mozilla.org/show_bug.cgi?id=373701
Put simply; if you visit an mjpeg stream then that stream will never terminate. No matter if you close tab, navigate away or whatever.
Firefox will continue to receive data on that socket and process it.
If you revisit the same stream or another, you get a SECOND stream, then a third - and so on.
This leads to:
1. Very high bandwidth consumption.
2. Very high CPU usage. (25% for each stream on my work pc)
3. Browser instability and the inevitable crashes that follow.
If you use mjpeg streams at all, I strongly urge you to stick with FF2.* (where the bug never existed) until a later version. I'm very disappointed with the mozilla team for ignoring what many consider a serious bug.