I think you are referring to one of the many drivers for connecting to MongoDB.
"MongoDB (from "humongous") is a scalable, high-performance, open source NoSQL database. Written in C++"
I do not know that spidermonkey is required, there was no mention of it when I installed MongoDB. I was able to get mongod to run without any specific action relating to spidermonkey. (this is actually the first time I've seen it mentioned)
I installed mongodb using wget and tar:
$ wget http://downloads.mongodb.org/linux/mongodb-linux-i686-1.6.4.tgz
$ tar xzf mongodb-linux-i686-1.6.4.tgz
$ mv mongodb-linux-i686-1.6.4 mongodb
$ mkdir -p mongodb/data/db
$ mongodb/mongod --dbpath ~/mongodb/data/db
Not too bad to install overall, however it would be very cool to have as a package.
I see mongodb is a java app and requires spidermonkey...
Edit: posted - let me know if it works