Fix Alt-d & F6 key to select urlbar

Note: untest on Max OS X.

References:
https://hg.mozilla.org/mozilla-central/annotate/2c9781c3e9b5/browser/base/content/browser-sets.inc#l254
https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly#w_miscellaneous
https://developer.mozilla.org/en-US/Add-ons/SDK/High-Level_APIs/system#platform
https://github.com/search?l=javascript&q=OS_TARGET+mac&ref=searchresults&type=Code&utf8=%E2%9C%93