webextensions-examples | Example Firefox add-ons created using the WebExtensions API | Addon library

Since you have the onGot(item) function working. You should give your preference_url a name to be traced from.

As you can see, when you inspect the [ Extension Storage ]. The preference_url is stored in a key named as type.

So, you should give it a name like preferred_URL or other name for easy tracking.

Quoting MDN:

To intercept resources loaded by a page (such as images, scripts, or stylesheets), the extension must have the host permission for the resource as well as for the main page requesting the resource.

So, you also need to add 127.0.0.1 in manifest.json because it's not the same as localhost, which can actually point to a different IP.

Very nice idea you had. But In short: no possible.

Main problem is that you cannot inject and manipulate on the homepage which is a built in browser tab.

"You cannot inject code into any of the browser's built-in pages, such as: about:debugging, about:addons, or the page that opens when you open a new empty tab." https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/executeScript

At first let me say that you did a good job in transferring data between web extension (Thunderbird add-on) and native application using native messaging. It isn't easy to understand it and set it up, but you managed to transfer required data with some tiny glitches you describe in your question.

... the resulting file contains double quotes (") at the start and the end of the file as well as escaped double quotes (\")

In the add-on you obtain raw email data as a string - console.log(typeof raw) gives string which you then pass to port.postMessage. Although the documentation says it takes JSON object representing the message to send, but it seems to accept single string value which is valid JSON according to some standards. In Delphi code you receive the message via STDIN and parse it using TJSONObject.ParseJSONValue into TJSONValue. It will in fact create instance of TJSONString. You can verify that by examining the value of jsonValue.ClassName. The problem with quotes arises when you use jsonValue.ToString which returns quoted version of the string that is basically the same what you had before parsing. Use the Value property to return raw string value.

Using jsonValue.Value alone will not help you with the encoding issue. The raw message data that you obtain from the e-mail client is in EML format. It conforms to RFC-822 and that means it is ASCII encoded, but it can contain arbitrarily encoded message parts (see your own sample EML). Since you only want to save EML file as is not taking any encoding into account, the best would be to transfer raw bytes of EML, but this isn't out-of-the-box supported by Javascript and native messaging API. Therefore I'd suggest you to send Base64-encoded data string to native application where you decode it into raw bytes that you can write straight to disk.

To encode raw message data as Base64 string in add-on use function btoa: