HTML5 (ES2) Editor preview differs heavily from quality in Firefox

Hello everybody,

Currently I try to do some ArchViz practice and wanted to export my Scene to HTML5 so others can view it online.
Using the “Preview Rendering Level” changer in the Settings my scene looks like this in “Default Mobile / HTML5 Preview”

but when exporting to HTML5 and launching Firefox it looks like this:

Why is the quality in the browser so much worse? And are there general guidelines to achive better quality in HTML5?