The only thing keeping xulrunner in tree is stable version of gnome-extra/yelp, so once that is handled, rest of the reverse deps will get masked with it.
Please act on this by porting to net-misc/npapi-sdk, punting USE="firefox", or punting the package.
I attempted to convert USE="firefox" for USE="nsplugin" (and build the npapi-plugin module) but it seems that npapi-plugin isn't actually supported by the build system at this time. Probably best to drop USE="firefox" entirely.
A simple conversion to NPAPI-SDK isn't viable as the firefox module(s) links to libxul.
Please wait. I am preparing a version bump which has a reworked firefox support. which is hopefully xulrunner free.
ping? only this package, and "yelp" left
+ 21 May 2012; Justin Lecher <firstname.lastname@example.org>
+ Drop broken firefox support for removal of xulrunner, #415039