Finally, I tried Firefox 4 beta.
Currently, I’m a bit frustrated, because almost all addons doesn’t work, but that’s not the point 
Firefox 4 introduces huge amount of great new features,
among them, possible, one of the best UI improvements is:
Group Your Tabs
More info about Group Your Tabs in this video:
It’s not that comfortable to click that button on top-right part of browser, especially on netbooks, laptops, etc…
So here is the shortcut to open it:
| Firefox 4.0b10 | Ctrl+Shift+E | 
| Firefox 4.0b7 | Ctrl+E | 
| Firefox 4.0b6 | Ctrl+SPACE | 
New beta-version – new shortcut. When will they stop changing it?
Have a nice browsing day!