I believe it's optional on the part of the application developers, but it's not ...

nandhp on July 24, 2015 | parent | context | favorite | on: Firefox nightlies for Linux are now using Gtk+3

I believe it's optional on the part of the application developers, but it's not optional on the part of the users. (Although it only happens if you have a compositing window manager, so that's one way to disable it.)