I believe tablet support is not yet completed. Please see this thread about using a test/development version now if you don't want to wait for the official release:

Why is firefox not supported on android tablets yet. If it is still under testing, why not allow users to test it on their tablets and give feedbacks

Thanks, any idea when it will be available? It is my favorite browser and I would like to have it on my tablet too?

I looked for this information for 15 minutes, and can't find it. It must be somewhere, but based on the six month gap between Firefox 10 for Android and Firefox 14 for Android, you might just want to take the plunge and try the "Aurora" version now. Assuming you have ample memory, I don't think you have anything to lose.

http://www.mozilla.org/en-US/mobile/aurora/

You can try the Firefox Beta channel where mobile Firefox 15.0 is on. https://market.android.com/details?id=org.mozilla.firefox_beta