[gentoo-user] Emergeing Firefox 35.0 breaks Ghostery ... fixable?

Hi,

emergeing Firefox 35.0 runs without problems...but:
The plugin Ghostery (blocks tracker) fails to start
and a reinstallations ends with this error message
(popup):

"Ghostery cannot be installed because Firefox cannot
modify the needed file."

Is this a proble of the plugin, a wrong permissions
setting via emerge/ebuild or a problem of my Gentoo 
box?

How can I fix it -- a downgrade to the previous version
(34.0.5-r1) is not possible, since one had decided to 
remove that version from the ebuilds ???

Best regards,
mcc