As described with bug #349660 dev-lang/spidermonkey and net-libs/xulrunner collide. Therefore there should be an option to use either to install dev-db/couchdb.
I've just submitted bug #349825 for disallowing parallel installation.
Mozilla team, is this feasible/a good idea?
Nope. Xulrunner is going away (eventually, hopefully); the dep should just be spidermonkey. Xulrunner should definitely not be used to provide a javascript lib anymore. Bug 366729 describes this issue better i think (as well as its resolution).
That's what I thought. Closing this.