If you have the Android 4.0 (Ice Cream Sandwich) developer option - 'Don't keep activities' enabled, Firefox for Android won't load websites. Keeping this Android system setting enabled is not recommend as it may also result in undesireable behaviour in other Android applications.

Follow these steps to disable this setting:

1. Open the Android settings app, scroll down and select and tap on Developer options
2. Scroll down the list of Developer Options and uncheck the checkbox for 'Don't keep activities'
3. If Firefox is running, quit it by returning to Firefox, tapping the menu on the upper right and choosing Quit.
4. You can then relaunch Firefox and it should behave normally.