Sometimes other applications may be conflicting with Firefox for Android. Try closing other applications to see if it helps. Please also check that you have the latest update from the Google Play Store

Thanks a lot for an answer but it did not help. I have the latest version. And only Firefox running. Still the same...

Please make sure that there is enough space for Firefox to run properly. Not enough space on the device may cause Firefox to not perform properly at it's best. Also, when you get the error message that Firefox is not responding, what options are available? Is there an option to "wait" for Firefox to respond instead of having to "force close" it?

I believe there is enough space. Though how much is enough? ) And, yes. There is Wait option as well. Still choosing Wait option provides no change. 5 sec later Activity Firefox is Not Responding reappeares. Am I the only one having this? )

You can try filing a bug at Bugzilla.