How to Use the Develop menu
Make it visible
If a site says you need another browser to view it, choose Safari > Preferences > Advanced and put a check mark next to “Show Develop menu in menu bar.”
If you’ve installed another browser on your Mac, it’ll be listed in Develop > Open Page With. Choose a browser from that menu to open the page you’re viewing in it.
Spoof a browser
Go to Develop > User Agent to see browsers Safari can pretend to be. This menu is helpful when the mobile version of a site plays HTML5 video, but the desktop one demands Flash.