* Unresponsiveness due to one tab thrashing
* Bad use of screen-space (bookmarks always visible, cannot disappear titlebar like in Chrome [there's only a Windows extension for that], menu-bar takes up another bar of space by default)
* Launching firefox has stop-the-world behavior to check/update extensions -- interrupting your flow when you need your browser most
* Some extensions still require restarts
These are the ones I remember off the top of my mind, I'm sure there were more...
I really tried going back to Firefox, but the cumulative pain of all of these made me go back to Chrome.