Created attachment 290989 [details, diff]
The mozextenions eclass hasn't been updated since 2007. The xpi_install function must be changed or a new install function must be added to the eclass to utilize the new standard install location for extensions which is /usr/lib/mozilla/extensions/<appid>.
This feature has been implemented a long time back and is documented here:
Mozilla bug entry requesting the feature:
Attached a patch which modifies xpi_install to do just this.
Created attachment 291003 [details, diff]
eclass lacks a maintainer tag.
Actually the patch is incorrect, appid does not need to be included it is the emid that is used to install. I will have to double check this to make sure we do not break enigmail before I am comfortable committing this to the tree.