No problem. I looked it up last week because I thought this was the release that was going to have the new fix for add-on-related memory leaks[1], but it turns out that's not coming until Firefox 15 next month.
Fullscreen also has security issues, more in fact, I believe? And it is not behind a flag. Also, mouse lock is enabled in Chrome in NaCl apps, but not HTML5 ones. So I am not clear on what's going on there.
It also exists in recent Chrome versions, however it seems to be disabled by default for some reason, but you can enable it in the flags menu.
No reason why this cool site can't use it on those browsers, there are lots of demos on the web that do. Mouse lock is very important.