Cannot install any browser

I'm currently on OS X Mavericks 10.9.5

A while back I switched from Safari to Opera because it was faster. But since a new update to Opera, it's laggy, and my laptop heats too much. So I wanted to try other browsers, but I can't install any ! I can download them, but when I click on the icon to install it, I get this message ( here it says Firefox, but it's the same message for any browser )