Test it our for yourself. Drag this button/link into your Firefox Bookmarks Toolbar Folder:
Here’s a common scenario to help you illustrate why this is so useful…
Say you are on blog.auinteractive.com and you remember that I wrote an article some time ago about finding cheap flights. Instead of hunting around for my search box (some sites have them some sites don’t) or having to go to google.com and putting in “site:blog.auinteractive.com ‘cheap flights’ ” as a query, all you have to do is click the “Google It” button in your toolbar – it’s always just 1 click away – type in your keywords in the alert box, and BAM, you get a Google search results page for what you were searching.
It’s basically the same as running a site: command, but without the extra steps and without the copy/pasting of the domain. Hopefully that’s a clear enough explanation of how this bookmarklet can be useful.
You can find more really useful Firefox bookmarklets at Stuntdubl’s blog.