Still using OS-X 10.6.8 on mid-2010 Mac Pro.
Firefox 48.0.2 works for many sites.
Occasionally use OS-X 10.5.8 on mid-2004 Power Macintosh G5.
Bought a MacBook Pro mid-2012 to try “new” operating systems. It runs Sierra okay on a spinning drive. Reluctant to go to High Sierra or Mojave.
Bought a MacBook Pro Retina mid-2015 to try “newer” operating systems. It runs High Sierra on Apple’s SSD. Reluctant to go to Mojave.
Have iPhone 8 running iOS 11.4.1 and reluctant to go to iOS 12.