According to their bug tracker, WebRTC is currently broken on Firefox Android. You can get audio or video, but not both, and attempting to get both results in your getting neither.
As for Chrome Android, it's only in the beta (M25 and later) and is disabled by default. To enable it, type
chrome://flags into the url bar, then scroll down and find the "Enable WebRTC" checkbox and check it. It'll then prompt you to restart the browser.
Note that WebRTC in Chrome is really beta, not "Google beta" and has some issues, such as;
- Audio sounds choppy and warbled especially on devices where the CPU is heavily loaded
- Acoustic echo cancellation on mobile platforms is not working well
- Chroma components received from the Galaxy S3 camera drivers are swapped (colors look inverted)
- DTLS via OpenSSL on Android fails and needs to be disabled
- General stability and complexity issues
Another note, neither of these browsers quite follows the standard, so the people who are writing the sites need to be adapting for the browsers eccentricities and if they aren't being throughout, it will work on one browser, but not the other.