This page lists SpiderMonkey release notes.
Note: Standalone SpiderMonkey is not an official product. Our continuous integration system does produce a tarball that is built into a binary that runs our smoke tests, but we do not maintain it nor actively test its suitability for general embedding. We have periodically created "releases", but they are best-effort and incomplete. We do happily accept patches, and make some effort to keep the tip of the Gecko tree minimally working as an embeddable source package. We are very limited in our ability to support older versions, including those labeled as "releases" on this page.
The easiest way to fetch the version corresponding to the current Firefox release is to visit the treeherder page for the release repository and click on the first SM(pkg) link you see. That will open a small window in the bottom left with a line like " mozjs-57.0.1.tar.bz2".