First, Unity3d only works on Firefox with pipelight, Chromium no longer supports the Netscape Plugin Api, which pipelight needs. In fact, Unity3d doesn't run on Chromium for Windows either.
The Unity javascript component (which may be embeded in your sites javascript sources) looks for the 'navigator.platform' value of your browser to contain either 'win' or 'mac'. Since no user agent spoofer for Firefox does this (I use 'User Agent Overrider 0.2.5 on Firefox 38 on Ubuntu 14.04.2), you must set your own override yourself.
Type 'about:config' in the Firefox address bar, get past the warnings and right click on the list of key-value pairs to creat a new string called 'general.platform.override' and set the value to 'Windows'
Here is the link to the test demo recommended by the pipelight maintainers
http://unity3d.com/showcase/live-demos#tropical-paradise
Be sure to run pipelight-plugin --system-check first to see if OpenGl is working correctly. if you see 'llvmpipe' as the OpenGl renderer, you will have a terrible framerate as xorg will be using software 3d instead of hardware. In my case it was because of a problem with xorg.conf. Disabling xorg.conf fixed the problem.