So I have been using firefox 4 for about a day now. (I don't have time to test trial beta's these days so only trial full versions)
My first impression was "damn that's quick to open"
Quickly followed by "damn where the hell is everything"
They have moved everything the tabs are up the top (you can move them below).
My bookmarks are over on the right hand side
To open a new tab it is now the top menu item not the second
There is a new orange firefox pull down menu which seems to be like offices round circle thingy that holds all the information. You can also pin tabs as well now. So no need for a plugin to do it.
About 40% of my plugins don't work and I have to recode all my personal plugins to work out why they aren't working or why they aren't being turned on by the system.
Honestly I don't like the new look but I am sure it will grow on me. I am sure there is either a skin or you can turn the look back in time.
The speed is impressive, not sure it's as fast as chrome but definitely a huge difference from 3.6 to 4. So far even firebug hasn't had a memory leakage. But then I haven't tried any really cool dodgy sites yet.
My overall thoughts, so far the speed is worth the effort of upgrading. If it doesn't have that problem with memory leakage then I think I will upgrade all my tools and then all my computers.