The latest versions of Safari probably work okay, but they are available only to users of the latest version of MacOS. Can't upgrade to the latest Safari without upgrading to the dreaded High Sierra.
That would be incorrect if you weren't running Yosemite. (There's no way for you to get a later version of macOS other than HS?)
As per
Safari version history...
OS X 10.11       El Capitan      11.0.2   (December 6, 2017)
macOS 10.12   Sierra              11.0.2   (December 6, 2017)
macOS 10.13   High Sierra     11.0.2    (December 6, 2017)
Which is not to say that I haven't had to switch to Firefox any number of times to access websites that Safari (the latest version at the time) couldn't reach...including to buy my daughter's new MBB, because Safari couldn't access the Apple Store. THE APPLE STORE, FOR CRYIN' OUT LOUD!!!
