Opinions later 
A picture might paint a thousand words so first, a handful of screenshots from Mozilla areas.
Two from May 2017:
– and three from this month:
If you ignore the four coloured buttons in the title bars of this month’s shots, you might think that there’s little resemblance to Mac OS X.
Short story
In the three years since I chose to abandon Apple products (with an adoration of software such as Finder and Safari), I have grown to use things such as KDE and Firefox on FreeBSD in ways that are very heavily influenced by the best of the Mac user experience of the past.
The five shots above might not remind you of macOS (or OS X or its predecessors), but there is a design essential that’s no longer possible with Apple’s software, and that essence was lost with 10.10 (Yosemite) …
Longer stories
… and those design mistakes drove me, after more than two decades of using Macs, to withdraw from AppleSeed projects before Yosemite was released.
(Apple is one of many open-, closed- and mixed-source backgrounds that I could re-throw into the introductions area … maybe next year.)
Human interface guidelines
One of the screenshots from May, and one from this month, show something that’s no longer possible with Apple software such as Safari.
A basic human interface guideline that Apple trashed in the transition from Mavericks to Yosemite.
Can you tell what it is? …