How to use Google Text-to-Speech? This might be a no-brainer but I am having a hard time figuring it out.

Today I got the update of Google Text-to-Speech on my HTC One X. I was able to configure it and even listen to an example. But how do I use it? The Play store description states that it can be used with Google Play Books, Google Translate and many other applications in Play Store.

But what if I have a page on my Firefox or Pocket app which I want to read it out? How do I use Google Text-to-Speech with that? Also this app does not show up in my app drawer. Any reason why?
Because Google Text-to-Speech isn't really an app: it's a library app. That is, it doesn't have a user interface for you to interact with; rather, it offers an interface to other apps to use its functionality. Other apps on the phone can give text to Google Text-to-Speech for it to speak out loud.

It's up to the author of the app in question to use the facility offered by Google Text-to-Speech: only apps that have been written to use it can do so. So if you want Firefox to use Google Text-to-Speech to read out web pages, you need to file a feature request with Mozilla (the organization that develops Firefox) asking them to support this.

For the same reason, the way you use it in an app that does support it, such as Google Play Books, is different from app to app. There might be a button or menu item to read text out, or there might be a setting to make the app read things out automatically.