The mozilla.org site includes file scheme test cases which should show you what formats Mozilla based browsers can cope with. Note that literal spaces are not allowed in URLs, and must be represented as %20.

I couldn't find such a reference. Where on that page does it say that spaces aren't allowed in URLs? Any modern browser handles spaces in URLs gracefully. After all, remember the original reason for encoding spaces:

The space character is unsafe because significant spaces may disappear and insignificant spaces may be introduced when URLs are transcribed or typeset or subjected to the treatment of word-processing programs.

I don't think that's a significant enough reason to forbid spaces in entered URLs (which can be replaced by %20 in the location bar and page properties, etc. later) compared to the benefits. And indeed I've never had a problem in any Mozilla-based browser involving spaces.