Firefox webextension Error: Could not establish connection. Receiving end does not exist

You get the error:

Error: Could not establish connection. Receiving end does not exist.

when you attempt to communicate (e.g. tabs.sendMessage(), tabs.connect()) to a tab where a content script is not listening for messages. This includes times when a content script does not exist in the tab.

You can not inject content scripts into about:* URLs

For your issue, you are getting this error because no content script is injected in the tab. By the time you are trying to send the message, in a main_frame webRequest.onErrorOccurred event, the URL for the tab is already about:neterror?[much more, including the URL where the error occurred]. You can not inject content scripts into about:* URLs. Thus, there is no content script in the tab listening for your message.

Specifically, you have used the <all_urls> match pattern in your manifest.json content_scripts entry. <all_urls> matches:

The special value "<all_urls>" matches all URLs under any of the supported schemes: that is, "http", "https", "file", "ftp", "app".

It does not match about:* URLs.

For a bit more discussion of the URL used when Firefox gets a webRequest.onErrorOccurred event in a main_frame, see "Injecting into navigation error page gets: Error: No window matching {“matchesHost”:[“”]}"

Another solution to this problem: If you reload your extension (as a normal part of the dev) it severs all connections to content scripts.

You must remember to reload the page with the content script as well for it to re-listen correctly.

I also had this same error.

My problem and solution was different but I'm adding it in case it helps.

In my case my content.js script initially did not have a browser.runtime.onMessage.addListener() function (FireFox).

When I later added this listener to the content.js script I did not reload the temporary extension in the "about:debugging" page in FireFox. I got the above error.

After clicking "reload" in the "about:debugging" tab the content script received the message.