SimilarWeb allows you use the extension interface for both browsing and voting instead of being limited to a web-based directory. Once you have the extension installed, using it is as simple as clicking on the SW icon beside the address bar. You can rearrange the suggested order of sites, submit new sites, and vote a suggested site up or down based on relevance.
The database is surprisingly extensive thanks presumably to a very active user and not a single site I tested was devoid of relevant suggestions. The SimilarWeb interface can be run from a sidebar if you desire, but it should be noted that the thumbnail previews of the suggested sites displayed there do slow your browsing down. If a 2-3 second lag is unacceptable, stick to using it as a toolbar helper. SimilarWeb is free and works wherever Firefox does.