Opening HTML5 YouTube videos (and possibly HTML5 video in general) make heap-unclassified numbers go up quite a bit. Closing the relevant tab does eventually make heap-unclassified go back to normal (although it might take a while), so this is likely caused by missing memory reporters somewhere. Interestingly, the amount of memory consumed seems to be related to the player size.
Can be reproduced with latest 32-bit Nightly on Windows 7 64-bit. Haven't done enough testing on other OSes or with older builds, but I'm pretty sure the issue has been around for some time and can possibly be reproduced on Linux as well.
Here are some of my measurements (done with Nightly 2012-04-11 win32 on Windows 7 64-bit):
19,363,448 B (16.58%) ── heap-unclassified (normal, after some light browsing)
31,602,376 B (22.44%) ── heap-unclassified (video open, small player size)
56,717,092 B (34.24%) ── heap-unclassified (video open, large player size)
1. Start Firefox and open about:memory. Take note of heap-unclassified numbers.
2. Open HTML5 YouTube video.
3. Update about:memory and take note of heap-unclassified numbers.
4. Change video player size to small/large.
5. Update about:memory and take note of heap-unclassified numbers.
I assume this is a WebM video, in which case this bug is likely to be a dup of bug 682216.