As it usually goes with high-profile new services such as Wolfram Alpha, Firefox add-on developers have quickly caught on, and there are already three quite useful WA-related add-ons in Mozilla's database.
First, there's the simple plugin that adds Wolfram Alpha to the list of search engines in Firefox. Even if you don't plan on using Wolfram Alpha on a daily basis, it's a good one to have because it makes it readily available without having to visit the actual site.
SlimSearch is a great, simple search plugin that lets you select text on any webpage and instantly send it to a large number of search engines. The latest version includes support for Wolfram Alpha, too.
Finally, the most interesting WA-related plugin is aimed at those users who really love the service; so much, they want to merge it with Google. The plugin (beware, it's experimental, and we have heard reports about it messing things up in Firefox) adds Wolfram Alpha results next to Google's results. Therefore, whenever you search for something in Google, you get the best of both worlds. This plugin is also ideal for comparing the results between these two engines.
More Wolfram Alpha resources from Mashable: