I'm finding it a mixed bag. The JavaScript performance improvements were SHOCKING as prior to the new engine Spidermonkey was anywhere from half to a fifth the speed of V8. Now with quantum it's on par if not faster. With the massive amount of scripttard bloat on websites, this is a welcome improvement as to be frank the old engine was aging like milk left out in the sun on a hot July day... it had turned green and there was a tree growing out of it.
The new "cleaner" interface is a bit... I dunno... That it put flexible spaces before/after the address bar didn't exactly wow me, but those were easily enough removed so that the FULL free space of the screen can show the URI. I do find I prefer the 'light' theme over the default, but that's just cosmetics.
"top sites" remains the same useless crap it has always been, naturally this gets replaced with a more Opera-like speed dial, sadly none of them are as capable as what Vivaldi does out of the box. Wow, am I an old-school Opera fan from before they told loyal fans to **** off or what?!?
I do not like how the old web developer toolbar add-on is no longer allowed to show as a toolbar, but is a tabbed based drop-down. This seems to be the mold now for add-ons and whilst I would like it as an OPTION, there ARE toolbars I use all the time since I would be a very hard sell to use FF as my daily driver. I'd be far more likely to embrace it if I could do simple things like portrait mode tabs out-of-box without goofy crap like "tree's" that the add-ons do. Again, Vivaldi user...
I really am NOT seeing any significant changes in the document inspector or other tools apart from moving stuff around for no reason other than to break muscle memory.
The pie chart on the network breakdown does seem to finally be fixed and not be dropping the ball on flat out refusing to see large amounts of loaded assets on a number of sites. Since the content it often missed was loaded via scripting, I would assume this change is part of the new scripting engine.
I do notice a LOT of <select> type menus in add-ons appear to be broken, the dropdown/flyout appearing at the bottom of the display and nowhere near the control.
It's still new enough that I'm waiting to see how bad things go bits-up face-down with it. I have an inherent distrust of FF in terms of memory hogging, instability, piss poor UI design... It has never been a browser I recommended for anything more than use as a development tool, particularly with how FLOSS fanboys gushed over everything about it and refused to admit its shortcomings -- to the point of "it's not a bug it's a feature" being their rally cry.
So they've got a lot of burned bridges to mend with me as a user.
I wonder if they fixed bugzilla 915, or will that one stick around until it is old enough to drink? Nothing like slopping in HTML 5, CSS3, and ECMAScript 6 when they don't even have HTML 4 and CSS2 complete.