[pcwalton]

(1) Sync is now on asynchronous APIs as of the latest beta (or maybe the next one). You can disable Sync in the Preferences window, and you can choose to only sync bookmarks if you'd like.

(2) Places (the bookmarks database) is now on asynchronous APIs, which means that, unless you're using esoteric features of bookmarks, they never block the UI thread. SQLite is also now in WAL mode. Smart folders are still slow as I understand it, and there's a plan to revamp and/or remove them.

(3) Bookmarks and history have been treated the same way since Firefox 3 (Places). There is talk of revamping and/or removing Live Bookmarks as well.

(4) Tab Groups is now secondary UI (the icon is gone by default). It is never loaded until you open it.

(5) The status bar is no longer in the address bar.