Firefox extensions/themes don't install

[Patrick Wiseman <pwiseman@gmail.com>]

It's probably a permissions thing - isn't it always? - but when I try
to  install Firefox extensions and themes (on debian testing with
firefox 0.9.3), most just fail silently.  I have a feeling this is a
FAQ, but Googling hasn't helped, and there's nothing obvious (to me)
at the extensions and themes sites.  Could someone just point me in
the right direction?

Thanks.

Patrick