The current plugin is able to detect your Firefox profile(s) in standard directories on Windows, Mac, and Linux (including snap-based Firefox profile directories) and will create a link to a custom user.js file in each profile directory the plugin detects.
When Firefox starts, it looks for a user.js in your profile directory, loads it if found, and updates its configuration using whatever values you specified. For example, you can standardize your Firefox homepage by creating a user.js file like this:
My own user.js file contains directives to customize my new tab settings and some telemetry settings. MozillaZine maintains a long list of configuration settings and descriptions of each.
If you're using dotbot and Firefox, use dotbot-firefox to standardize your profile configuration!