Firebug is a Firefox plugin that lets you inspect the code on a webpage, giving you a quick way to check if best practice, SEO-friendly coding has been used. The ability to edit page code and content means you can create 'before and after' images of changes that you suggest to a site.
Something that I didn't mention in the video, is how useful the app can be for helping people who've not spent a lot of time looking at HTML to understand the structure of pages and the purpose of different HTML tags and CSS code.
Other useful features include a tool to monitor all the http requests made by a page, which helps identify any linked images or external code files which are slowing down page load speed.
Firebug is one of the most important SEO plugins for Firefox. It's an invaluable part of an search marketer's toolkit, and I'd highly recommend having it close at hand.
NB: this is the first video blog post I've done. Let me know if you have any thoughts about the format. I used the incredible usability testing tool Silverback App to create the video.