Add note that Firefox uses the wrong interface for <kbd> (#740)
Add note that Firefox uses the wrong interface for <kbd> (#740)

prior to Firefox 4. Only applies to Desktop, since the
first mobile release was after this.
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,teoli2003/browser-compat-data,chrisdavidmills/browser-compat-data