Found the information I had read about Firefox from the Unreal documentation:
With Mozilla, game can be launch in offline
Mozilla can bring large, high-performance applications to HTML5 and 64 bit browsers can allocate more than 512MB of memory to around 1.5 GB.
For things to work on chrome x32 & Chromium x64
the files need to hosted behind a web server, chrome doesn’t like direct file system access.
However, this page was updated last in March 2015, so I assume many things have changed since then.