Problems with addons in Firefox

[tribunal]

After installing Kubuntu Feisty, I can't install addons in Firefox
everything I see is "error 203"
I have found (during googling), that I should start firefox with sudo (as root)
But I can't
sudo firefox just doesn't work:
Xlib: connection to ":0.0" refused by server
Xlib: No protocol specified


(firefox-bin:15263): Gtk-WARNING **: cannot open display:

Any ideas?
(2 problems: addons and starting firefox as root)

[tribunal]

Re: Problems with addons in Firefox

Solved!
Problem is in old ~/.mozilla directory (it was not cleaned after installation Kubuntu over openSUSE)