To be fair, it's incredibly CPU-heavy. It uses an iterative approach to calculate the forces exerted on the grid, so for each frame it calculates the force exerted on each node a set number of times. (See http://en.wikipedia.org/wiki/Verlet_integration)
However, this shouldn't be so heavy that the Firefox JS engine struggles with it...
Yeah the weird thing is that aside from being unresponsive it seems to run fine. The lines move around and the fabric stretches and waves smoothly. That's on an older i3, and a newer 3.GHz i5. I bet it works properly in chrome. I like Firefox but I always seem to have performance issues with it.
However, this shouldn't be so heavy that the Firefox JS engine struggles with it...