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.
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
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.