The title "How to resend webrequest via a firefox extension?" is a bit misleading. It should be something more like "webrequest handler extension doesn't load," since that's the actual question.

I know it's been a while since you asked the question, so maybe you already know the answer, but you said:

The Problem is, I dont even see the "Background script loaded" log at the beginning!

I didn't debug your code, but when I load it as a temporary add-on, I do get "Background script loaded" logged to the console. Make sure you're looking at the right console. On about:debugging > This Firefox, "Inspect" your extension. That's the console where your background logs console messages.

One other thing I'll point out: the extraInfoSpec parameter for addListener doesn't have valid "extraHeaders" and "requestHeaders" values. There would be an uncaught error if you try to load your code with those values. The error shows up in the console, too.

  ["requestBody", "blocking", "extraHeaders", "requestHeaders"]

should be just

  ["requestBody", "blocking"]