This text-to-speech Firefox add-on works well but the bookmarklet version is even better!
Anyway, I had a play with two solutions today, both based on vozMe's text-to-speech API. The first is a Firefox add-on that sneakily asks for contributions while not even crediting vozMe! Disgusting -- but still, the add-on works well. You can select blocks of text and convert it via the right-click context menu, or click the speaker in the bottom right corner of the browser. Right-clicking the speaker gives you a lot of options too: there are a lot of international languages available, and male or female voices!
The second solution, vozMe's own bookmarklet, is better for quick-and-dirty work. Bookmarklets are both more efficient and work with every browser. If you click through the languages in the top right corner, you can also create male/female bookmarklets for a variety of languages.
It's worth noting that the Firefox add-on also lets you download an MP3 of your text-to-speech clip! Also, the transliteration isn't very good -- some of vozMe's attempts at pronouncing English placenames were dire to say the least.