Originally Posted by XavierP
What does this mean? The fact it's installed to root's home tells me that you installed as root (using su?). Install as your normal user without admin rights.
Sorry about the confusion, of course I used to install extensions as normal user without sudo, and they work except this one. I just said that this user account has 'User privileges' to 'Execute system administration tasks'. I installed the plugin with 'sudo firefox' only *after* the normal user installation hasn't worked: Firefox couldn't start first, then on the second restart the plugin was there but broken. *After* this tried to run firefox with sudo and install the plugin, and it worked, but obviously the plugin is in the root's profile and I won't run Firefox always as root because of this.