(Notice the Add-on Bar at the bottom)
Add-on Bar vs Statusbar
Just when I though Firefox 4 is finally coming together, the latest Firefox 4 nightly brings a so called "Add-on bar" which is just like the old statusbar which Mozilla has been trying to deprecate with the recent changes (links and progress bar in the location bar), only bigger.
If you think it might look so big because of the theme (I though so too), you'll notice it's just as big using the default Ubuntu theme (just make sure you add something to it as it's smaller when empty). Fortunately, in the same update the old statusbar was completely removed and this new Add-on Bar doesn't come enabled by default.
If you want to enable it in Firefox 4 (lastest nightly build), simply go to View > Toolbars > Add-on Bar:
New Stand-Alone Profile Manager
The recent Firefox 4.0 builds come without a profile manager (so running Firefox with -profilemanager in Linux doesn't work anymore). The cause for this is a new stand-alone profile manager which should be introduced soon.
The new profile manager will allow you to manage multiple profiles easily including temporarily profiles - great for testing new Firefox versions. Also, you'll be able to choose which Firefox version to use (Manager Firefox Versions > Add) and for which profile so running multiple Firefox versions will become very easy.
This new, experimental profile manager does not come with Firefox for now but you can try it out by downloading it from HERE - available for Windows, Linux and Mac OSX (to use it in Linux: extract it and double click "profilemanager-bin").
See also: Firefox 4 For Linux Video / Screenshots Preview (Beta 7 Pre)
Info in this post partially thanks to lovinglinux (creator of FoxTester).