The HTML client no longer works in Firefox on our Nexus 7. It has worked previously though. One guess is that the upgrade to Firefox 25 was the cause. Firefox 25 on Samuel's phone worked fine though.
The symptoms are that the browser page stops rendering when the VNC connection becomes active. The rest of the UI is still responsive though. It even updates the preview in the tab switcher properly.
This seems to be a browser bug as the page remains frozen even when you navigate to a different page. Rotating the screen seems to unfreeze it.
Likely the same problem: https://github.com/kanaka/noVNC/issues/321
Fixed by upgrading Firefox.
Couldn't reproduce, Nexus 7 using Firefox 37.0.2