Cookie Editor & Adscript replacement spoofer add-on

Here's my problem: I work in Digital Media, Ad Operations, trafficking Digital display ads. Generally the Ad tags are in Javascript or i-frame format. Our headache is creating screenshots of the ads in a live format, capturing screenshots with a screen capture tool, for verification to send back to the Ad Agency or client. We know where the ads are served from our publisher reports. We end up usually mocking up a screenshot by capturing the website page, capturing the ad element in a different environment, and using an image editing software like Snagit to overlay the ad in the right spot. Typically this task takes us about 30 minutes to create a set of screenshots. What we need, is a tool to edit the adscript tags, or replace the adscript on a designated website. In other words, we want to "spoof" the website and create a live mock-up of the site so we don't have to cut and paste. Ideally, this would be a tool we could add on to Firefox or Chrome. Across our company, it would save us a couple hours each day and allow us to focus on more added value tasks, versus doing copy / edit / paste tasks.