Back in December of last year, we released a Chrome extension called Toolbox for Google Play Store. It enhances the web Play Store with testing program signup links, a button to view the selected app on APKMirror, and more. A few months later, the extension became available for Opera, and now it's on Firefox too.
The Firefox extension uses the same codebase as the Chrome and Opera versions, so you don't have to worry about missing functionality. The Firefox version is a WebExtension, which means it's fully compatible with newer browser features like Electrolysis. It also means that Toolbox will continue to work after Mozilla drops support for legacy add-ons in November (with the release of Firefox 57).
You can download Toolbox for your browser below. If you found a bug or have a feature request, you can leave them in the comments or on the issue tracker.