Now you can install global-menu for Firefox as an extension is in development that brings support for the browser in Ubuntu 11.04.
The extension is available in a PPA for Ubuntu 11.04 only and you require latest version of Firefox 4 - beta 9. To install the extension, run following commands in your terminal:
sudo add-apt-repository ppa:chrisccoulson/ppa
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install firefox-globalmenu
Installation instructions will be changed overtime so make sure that you keep yourself updated with Ask Ubuntu page here. Also, the extension is not yet complete so if find any bugs you can report theme here.
[screenshot and info via Jorge Castro's Blog]