The <details> element is not currently (11th May 2016) supported by IE and is an experimental feature in Firefox requiring a preference ‘flag’ to be set/enabled.
This feature is available since Firefox 47 behind the preference dom.details_element.enabled, defaulting to false, except on Nightly and Aurora versions (bug 1241750). Support for it is enabled by default, i.e. the preference is defaulting to true, since Firefox 49.0 (bug 1226455).
Regarding the font issue (and please don’t ask more than one question at a time) I believe the other answer is correct…use quotes.
solved Unordered List and fonts appearing differently in different browsers