It looks like some of the user experience ideas from Google Messenger have made their way to Google Hangouts. How long does Google plan on supporting two Android SMS/MMS apps?
I've been using Messenger and it's pretty nice. I especially like the ability to search through my messages, something that the Hangouts v4 screenshot doesn't show.
This would be useful if you were making UI components and you want a generic wrapper element to contain say a checkbox and a label. The wrapper can have display:contents, allowing the checkbox and label to have a shared parent for movement through the DOM or selection via JS, but otherwise have no styles that apply to their parent.
You should go to about:support and use Refresh Firefox which will probably fix most of your crashes (outside of Flash-related ones which should only be affecting the plugin and not the browser).