I'm using "Mozilla/5.0 (Windows NT 5.1; rv:24.0) Gecko/20100101 Firefox/24.0 ID:20140131092626 CSet: d06a17a96fa2" (actually version 24.3.0), and when I try to view the spec (from the link on <https://developer.mozilla.org/en-US/docs/Web/API/Selection>), I get a very understyled page.

The problem becomes (reasonably) clear if we look at "Developer Tools: Web Console":

[21:56:19.480] GET https://dvcs.w3.org/hg/editing/raw-file/tip/editing.html#selections [HTTP/1.1 200 Script output follows 641ms]
[21:56:19.878] Blocked loading mixed active content "http://www.whatwg.org/style/specification" @ https://dvcs.w3.org/hg/editing/raw-file/tip/editing.html#selections
[21:56:20.381] Blocked loading mixed active content "http://www.whatwg.org/specs/web-apps/current-work/dfn.js" @ https://dvcs.w3.org/hg/editing/raw-file/tip/editing.html#selections
[... skipped irrelevant deprecation warning caused by some extension ...]
[21:56:23.642] GET http://www.w3.org/Icons/w3c_home [HTTP/1.1 200 OK 16ms]
[21:56:23.653] The Web Console logging API (console.log, console.info, console.warn, console.error) has been disabled by a script on this page.

(Actually, this pasted text is clearer than what was on the screen ...)

So, it seems Firefox decided that it's had enough of people trying to use "http://" resources from an "https://" document and finally put its foot down.

Unfortunately, the obvious workaround (delete the "s") does not work because that redirects back to the https:// address :-(.