Firefox 4 is just around the corner and I am using this extensively since the first beta's. There are so much under the hood improvements like speed, support for HTML5, CSS3, hardware acceleration, crash protection, WebGL and many many more.
One other more visible feature I really like is "Pin as App" which gives your favorite website a prominent place on the tabbar with your other tabs. Together with also a new feature "Panorama" this is a very effective way to organize your daily life on the web or even better your work. If you, like me, have multiple projects going on for several days or weeks, you will end up with lots of open pins and tabs organized into labeled sections in Panorama.
Recently I had 2 issues with this type of organization. Firstly my Vista decided to give up (not by Firefox though) and pushing the power-button was the only way to restart. Firefox wasn't able to restore my open tabs. Secondly I was in a hurry and quickly exited all Firefox windows with the last window only open with Gmail. As my setting is to open with the latest session Firefox opened with Gmail again as supposed to.
Of course with lots of open tabs and pins this wasn't the behavior I like, which led me to the idea that if it is becoming that important, Firefox needs a Save-button. Hitting this button every now and than while working, will secure your current state and not the state after a crash or stupid mistake, just like you would do in any other document.
It is kind-a-like the add-on Save session with an option to restore. Will think about this a little longer.