At work we have to use a proxy which requires authorization. With previous versions (firefox 3.0.x and 3.5.y for each valid x and y) I had the problem that each tab requested to enter the master password when starting firefox, to be able to fill in the proxy-auth data (shortcut: fill in only the first request, and for all others just hit return/OK). So for each tab I had to do something for the master-password, and after that for each tab I also had to confirm the proxy-auth stuff.
Very annoying! Oh, I should maybe mention that as of this writing I have 31 tabs open. Sometimes there are more, sometimes there are less.
Now with firefox 3.6 this is not the case anymore. Yeah! Great! Finally only one time the master password stuff, and then one time the proxy-auth stuff, and then all tabs proceed.
It took a long time since my first report about this, but now it is finally there. This is the best improvement in 3.6 for me.