Designing a Firefox Extension to Save GET Responses with Contextual Metadata
Introduction
Web developers and power users often find themselves needing to capture specific network responses—particularly images—that are not easily accessible through standard browser features or existing extensions. While tools like Firefox’s Developer Tools provide powerful network inspection capabilities, automating and streamlining this process can save considerable time and effort. This article explores the process of creating a custom Firefox extension to automatically save large GET response images along with relevant contextual information, catering to users who want selective, metadata-rich downloads without automated crawling.
Understanding the Challenge
Many websites dynamically load images or serve them in ways that complicate manual saving. Typical issues include:
- Images embedded in non-standard formats or obfuscated.
- Default filenames that offer little context.
- Inability to easily re-save images via right-click or straightforward download.
- Existing extensions either download all images indiscriminately or do not provide contextual metadata.
In this case, the goal is to manually select images of interest and save them with associated data such as the source URL and possibly additional HTML context, enhancing the traceability and usability of the images later.
Proposed Solution
The ideal workflow involves a lightweight Firefox extension that:
- Runs in the background during browsing.
- Allows the user to signal when they want to save a particular image.
- Captures the most recent large GET response (above a specified size, e.g., 100 KB).
- Saves the image file locally.
- Includes relevant metadata, such as the source URL and supplementary HTML information, either embedded within the image or saved separately alongside it.
Implementation Overview
- Understanding Firefox Extension Architecture
Firefox extensions (WebExtensions) are built using standard web technologies—JavaScript, HTML, and CSS—and primarily operate through the WebExtensions API. For network interception and response handling, the webRequest API is key.
- Intercepting Network Responses
Use browser.webRequest.onCompleted or onHeadersReceived events to monitor network activity. To specifically target image responses:
- Filter requests by resource type or by URL pattern.
- Check response headers and size.
Note: Due to security restrictions, response bodies are not accessible through standard WebExtensions APIs. To access response content, the extension can inject content scripts or leverage the webRequest API’s blocking and responseBody features (if available).
- Capturing Response Data
As of recent Firefox versions, the
