TypeError: [API] is undefined in content script or Why can't I do this in a content script?

Question

I was trying to write a simple extension in Firefox wherein I modify the X-Frame-Allow header.

I looked at the documentation briefly and I see that they

Answer 1

This question led me here. I had a problem with a browserAction. This line was in my background.js for handling a click on my extensions' icon:

browser.browserAction.onClicked.addListener(handleClick);

That line gave me this error:

TypeError: browser.browserAction is undefined

What was the problem? I just forgot to define a browser_action in my manifest.json:

"browser_action": {
    "default_icon": "my-icon.png"
}

Maybe this hint could be helpful for someone else… ;-)