BuiltWith is a wonderful service to find out what websites are built with, and there is already an add-on showing the results in-tab.
As a developer, I frequently visit websites in bulk to find out what technologies are used behind them. Because of that, I prefer to have the BuiltWith results in the background tabs for me to check them out later in batch. This way, I don't have to wait for the results and can continue to browse the websites with no interruption.
Required FireFox 29+