Some of the libraries in epiphany-extensions, which link against mozilla, do not set the rpath/runpath tags, which could lead to problems by dynamic resolution (this will depend on ld.so.conf order of the exported directories - mozilla or firefox comes first, or will completely break the plugins, if there is no exported path there). There is also an attached patch, which resolves the problem. There is a possible problem only with net-monitor, which I could not build because of the dbus-dependancy and which could probably also link against the mozilla-libraries, in this case there should be added a line in its Makefile.in similiar to the ones in the patch.
Created attachment 64623 [details, diff] epiphany-extensions-rpath.patch This patch will set the correct rpath/runpath tag of the libraries, which link against mozilla.
Created attachment 65916 [details] epiphany-extensions-1.6.4-r1.ebuild That's the full ebuild, which uses the patch (adds only one epatch-line). The Net-monitor extension does not use mozilla at all, so the patch is fully working.
This bug is now obsolete with the new mozilla and mozilla-firefox ebuild, which will set automatically the rpaths needed.