Firefox is using the newer WKWebView, unlike Chrome.
Here is the relevant Chromium issue tracking the move to WKWebView: https://code.google.com/p/chromium/issues/detail?id=423444
That link also lists a hand full of Radars filed which are preventing the iOS Chrome team from moving to WKWebView.
Of course, there might be a difference between UIWebView and WKWebView, and I don't know which Firefox is using.