It is my understanding that it is already enabled. I have checked the aquarium out with Firefox 5 right now. Works perfect. It was in Firefox 4 that you needed a command to activate WebGL (Although the programs ran horribly). On Firefox 5 WebGL is much better. Not as good as Chrome of course but it is catching it up very fast.
If you are still having problems seeing that webpage it is because of a direct problem with your video card. Does your video card support opengl 2.0.
Here is a mini guid that points from what version the browsers support WebGL and what to do to make it work if it is in Alpha/Beta with the browser: http://www.khronos.org/webgl/wiki/Getting_a_WebGL_Implementation
In the Wikipedia: http://en.wikipedia.org/wiki/Firefox_4 you can see that in Firefox 4 it got some support for WebGL (But trust me, most WebGL examples from Chrome Labs tested horrible)
In the Wikipedia:http://en.wikipedia.org/wiki/History_of_Firefox#Version_5.0 you can see that Firefox 5 got more support for WebGL that now actually lets you play most WebGL apps. There are some even that are very tough to support and it supports them like http://www.ro.me/ "3 Dreams of Black" which is literally you flying inside a video thats playing cool music.