Firefox 29 uses the following HTTP Accept header in a normal request:
text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8,application/json

This results in Stash responding with application/json and a json body instead of the expected text/html login page. Since both text/html and application/json has the same default quality value, q=1.0, it defaults to Json.