Assuming that acorn's dependencies have not changed, to upgrade our tree's acorn to a new version: 1. Clone the acorn repository, and check out the version you want to upgrade to: $ git clone https://github.com/marijnh/acorn.git $ cd acorn $ git checkout 2. Make sure that all tests pass: $ npm install . $ npm test If there are any test failures, do not upgrade to that version of acorn! 3. Copy acorn.js to our tree: $ cp acorn.js /path/to/mozilla-central/devtools/shared/acorn/acorn.js 4. Copy acorn_loose.js to our tree: $ cp acorn_loose.js /path/to/mozilla-central/devtools/shared/acorn/acorn_loose.js 5. Copy util/walk.js to our tree: $ cp util/walk.js /path/to/mozilla-central/devtools/shared/acorn/walk.js 6. Check and see if javascript pretty-printing work without any errors. As of version 2.6.4 we need to comment out lines in acorn_loose.js that attempt to extend the acorn object, like `acorn.parse_dammit = parse_dammit`.