1. Sync slows down the interface of the application a lot. There should be an option to completely disable Sync. Or Sync should have been an add-on installed by default. If I want to sync only the bookmarks, do just that, nothing else. Sync should also be activated only when I save a new bookmark, not every now and then.
2. SQLite slows down the application and complicates things with those default smart folders. You can't even create a smart folder for bookmarks. Mozilla should have write a faster database for bookmarks and history — they are just soem lines of text after all.
3. Live Bookmarks are useless now since the RSS icon was removed. It was useless before. I think by removing the code for Live Bookmarks and treating bookmarks and history in the same way will make things work faster.
4. Tab Groups also slows down the interface. It is useless since you can manage multiple tab groups by opening new windows.
5. Moving the status bar in the address bar was a bad idea. It complicates the user experience. Status bar should be shown only when something new happens — a download stars, a new page is loading, etc.
I don't know what happens under the hood, but Firefox is heading in the wrong direction.
(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.
