Cannot install Firefox extensions

[Kevin Krumwiede]

I cannot install any extensions in Firefox, even when I run it as root
under sudo -s -H.  When I click an extension, it asks if I want to
install it, but when I click yes, the extension manager pops up and the
extension is not there.  (And it remains "not there" even after
restarting Firefox.)

Is this related to bug #4395? 
https://bugzilla.ubuntu.com/show_bug.cgi?id=4395

Thanks,
Krum