From what I can see in the WeiDU sources, readmes are started on Windows via "start" command.
You can test if this is indeed the culprit by starting a command prompt, go into the readme directory and call "start readme.html" (or whatever the readme filename might be).
To test file association itself you can execute "assoc .html", followed by ftype with the filetype returned by assoc (e.g. "ftype htmlfile"). ftype can also be used to set a new association. I haven't used it myself yet, so I don't know if there is more involved. It's probably easier to set Firefox as the default app for html from within the Firefox options.