My 2013 Macbook Pro keeps crashing with Firefox, a few times every day. I cannot use Firefox anymore, I had to switch to Chrome. And as I am writing this Mail crashed because I wanted to forward an Email with a large picture in it. And messages has crashed numerous times, I don't use that either anymore.
So yes, for me Apple's software quality truly has declined sind Snow Leopard.
I was probably unclear: The whole computer freezes, and there is nothing I can do except a "hard" reset, i.e. pressing the power button until it shuts off.
But there are more issues still, e.g. that the computer will only connect to my bluetooth radio after a restart.
Firefox is nowhere related to Apple of course, but it is so bad on Mac. I recently went back to Chrome because it just works so much better on my macbook. (El Capitan)
So yes, for me Apple's software quality truly has declined sind Snow Leopard.