Haven't really noticed any issues on Firefox except the lack of actually good web translator.
-moz-element looks sweet, hope it comes to other browsers.
But it's such a dog. Even after turning off mail and feeds which just sit there eating CPU like candy, I just can't make it fast, and returning to and cleaning up my Firefox made it so snappy, I'd rather stick to my 3 profiles (normal,dev, media) and do the rest via Bookmarks. And for all the sweet UI options and configurability Vivaldi has, with Tree Style Tabs and some addons for it, and hacking the UI CSS, I am sheepishly making Firefox my primary again, pretending none of that happened.
I wish it cached compiled WebGL shaders though. That is the only thing where Chrome beats it to a pulp sadly. Maybe there are config tweaks, I haven't looked into it, but out of the box reloading a page with a lot of shaders is basically instant in Chrome, even on mobile, and can take ages on FF, depending on the shaders.