According to https://bugzilla.mozilla.org/show_bug.cgi?id=686692 and https://bugzilla.mozilla.org/show_bug.cgi?id=687766 firefox no longer depends on libIDL. From the second bug report this seems to have been fixed in firefox 9, which means that we no longer have any ebuilds in the tree that need libIDL. I tested this by unmerging libIDL, removed it from mozconfig-3.eclass and reemerged firefox-17.0.2 without any problems. I also did a quick grep for libIDL in the firefox-10.0.11 sources, which gave no valid results either Reproducible: Always
Thanks for reporting dep has been drop'd