Speed; UI; Sync; Ability to do page screenshots;
Found several websites which work great on Chrome but have issues with Firefox;
Have been using it for some time now and migrated everything from Chrome. The Firefox Quantum features a great speed improvement, indeed, and a sleek new UI. (Look at the update below). However, I don't really like that the address and the search bar aren't taking the whole empty space but are rather constricted in the center of the toolbar. This creates some awkward blank space around them and on the right side, it feels like there are several missing toolbar icons. Everything else I've seen about the UI feels great, especially the tabs, the colors and how unobtrusive the new UI is in the browsing experience.
Update: As others have pointed out in the comments, the whitespace around the toobar could be removed and the toolbar icons can be customized so this is not an issue.
It might be me, but I have also observed some issues with pages rendering (the Firefox install I'm currently running is clean from ad-blockers and such plugins). For example, when I open a startup page on BetaList, the main image is never loaded. It might also be a compatibility issue of this particular websites.
I especially like the sync feature and the ability to easily make page (or element) screenshots directly within the browser and without looking for thrid-party extensions.