How to count HTTP responses per web page using WebExtension

Question

Using WebExtensions, I am trying to write a content script which would count the number of HTTP responses per web page.

I just started looking into WebExtensions, so I may be completely off the track, but so far my approach has been to modify the background script to listen to browser.webRequest.onHeadersReceived. Once fired, callback to that function would invoke browser.tabs.sendMessage to the tabId from where the request to this response originated.

In the content script I use browser.runtime.onMessage.addListener to listen to those messages from the background script.

With this approach I have two problems:

1. The background script is trying to send messages to the content script even before the content script has started listening, resulting in errors from the sendMessage and thus lost information.
2. On page reload, some of the messages from the background script are received before the content script is reloaded, some are lost (presumably during the period where one content script was unloaded and before another one started) and some are received after.

I've been looking whether browser.storage.local can help me with this situation. With this I can avoid the problem of messages being lost by simply keeping the count in that storage.

But I still have the problem in the background script that I don't know whether to increment the count for the web page that was displayed before the reload happened, or the one after (due to the problem #2 above).

I think instead of using tabId as a "pointer" to where the count is increased, having some kind of an ID for the web page to which the response belongs to would help. But I haven't been able to find anything like that in the documentation yet. But again, I'm novice to WebExtensions, so there may be a completely different approach I'm missing.