Chrome content scripts aren’t working: DOMContentLoaded listener does not execute

You are injecting your script after the event you are listening for fires (in this case, DOMContentLoaded). Thus, any code that you have in the listener will not be executed, because the event never fires after you have added your listener.

In Chrome extensions and Firefox WebExtensions, when specifying a time for a content script to be injected, you can specify "document_start", "document_end", or "document_idle".¹ In a manifest.json this is the value stated for the run_at property. For tabs.executeScript(), it is the runAt property.

• document_start
  The injection takes place prior to the DOM being created, or scripts from the page being run. This means that document.body and document.head do not yet exist. The DOMContentLoaded and window load events have not yet fired. You can add things to the DOM by adding them to the document.documentElement. You may need to use a MutationObserver to watch for the elements you are interested in being added to the DOM, or wait for an event like DOMContentLoaded to indicate that the DOM is available.
• document_end (default)
  The injection takes place after the DOM is complete, but before subresources (e.g. images and frames) are loaded. This is usually after DOMContentLoaded has fired, but before the window load event fires.

• document_idle
  The injection takes place sometime after document_end and immediately after the window load event fires. The answer to “When does a run_at: document_idle content script run?” indicates that this is the earlier of:

  • After the window load event fires, or
  • 200ms after the DOMContentLoaded event fired.

  This means that your content script will be injected after DOMContentLoaded has fired, but the window load event may, or may not, have already fired.

When listening for DOMContentLoaded, or window load, you should check document.readyState first

Any time you use a DOMContentLoaded listener, or a window load listener, you should always check the document.readyState prior to adding the listener to make sure that you are adding the listener prior to the DOMContentLoaded event being fired (or prior to the load event being fired, if that is what you are listening for). This should be normal habit when you want to listen for these events. If you add the listener after the event has fired, the listener will never be run.

For adding a DOMContentLoaded listener, you should use something like:

if(document.readyState === 'loading') {
    document.addEventListener('DOMContentLoaded',afterDOMLoaded);
} else {
    afterDOMLoaded();
}

function afterDOMLoaded(){
    //Everything that needs to happen after the DOM has initially loaded.
}

For adding a window load listener, you could use something like:

if(document.readyState !== 'complete') {
    window.addEventListener('load',afterWindowLoaded);
} else {
    afterWindowLoaded();
}

function afterWindowLoaded(){
    //Everything that needs to happen after the window is fully loaded.
}

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1. If you are using tabs.executeScript(), the value that you provide for runAt only indicates the earliest you want the script to be injected. If you are executing tabs.executeScript() prior to that time, then the injection is delayed until the specified time. Note that for document_start the point when executing tabs.executeScript() will be valid for a new page is a complex topic, which deserves its own question/answer.
2. Portions of this answer were copied from my answer to “Detect and handle a button click in the active HTML page”.