You must read the disclaimer before downloading.
Unlike Fire Vox which is designed for visually impaired users, CLiCk, Speak is designed for sighted users who want text-to-speech functionality. It doesn't identify elements or announce events - two features that are very important for visually impaired users but very annoying for sighted users. It also has a simplified, mouse driven interface that is designed to be easy for users familiar with point-and-click graphical user interfaces. Like Fire Vox, CLiCk, Speak works on Windows, Macintosh, and Linux; and Fire Vox has multilingual support, making it great for users who are trying to learn a foreign language and need to hear foreign language web sites read out to them for practice.
If you're a sighted user who wants to have web pages read to you because you have cognitive issues (for example, dyslexia), because you have literacy issues (like me - I can understand spoken Mandarin Chinese just fine, but reading is difficult for me), because you want to reduce eyestrain and listen to a web page being read, etc., then you are likely to prefer CLiCk, Speak over Fire Vox.
The Core Library Components for Text-To-Speech (CLC-4-TTS) Suite is a set of extensions for Firefox. It consists of a library for doing text-to-speech synthesis (the Core library), a library for navigating the HTML DOM (the Utils library), and an application that uses the libraries to act as a screen reader (Fire Vox).
While it currently only works on Firefox, it would be possible for me to create versions of this for other Mozilla products such as Thunderbird. I would have to rewrite the application portion, but the libraries would largely remain untouched. These libraries can also be used to construct other extensions that are not necessarily aimed at the visually impaired. Both are directions that I am interested in pursuing in the near future, so stay tuned.