I want to log ajax errors using New Relic's browser logger. However, nowhere on the website I can find relevant information about how to install it. Where do I get the nr.js file, and how do I start using it?
New Relic's browser monitoring is not a standalone JavaScript file; it requires a New Relic application monitoring agent (sometimes called a language agent) to be installed in the server-side application serving the end-user pages. Static pages served directly by a webserver (such as Apache), or purely client-side (frontend-only) applications, can't report browser data to New Relic, because there is no way to connect their reporting to a monitored application.
If you do have a New Relic-monitored server-side application, information on enabling browser monitoring can be found at https://docs.newrelic.com/docs/browser/new-relic-browser/getting-started/browser-settings.