Test it for yourself:

1. Install Firefox

2. Open Firefox and solve a ReCaptcha

3. Install Chromium

4. Open Chromium and solve a ReCaptcha

Notice the difference between Firefox and Chromium?

Google's ReCaptcha is gracious with Chromium/Google Chrome. You can even make mistakes and it still valid. With Firefox (and maybe other) browsers on the other hand... why not increasing the difficulty/challenge for them? -.-