Are you using Private Browsing mode or use Clear Recent History to clear items like the cache?

You are in Private Browsing mode if you see "Tools > Stop Private Browsing".

Yes that's solved the problem, but it's a workaround, it's not the complete solution because in older versions as 3.5 I could be in private browsing and in the same time to get always these icons in the personnal bar. So something has been changed in version 3.6.3 about the place where are copied these favicons compare to the version 3.5 and under. The best solution would be to get again the properties of the version 3.5 about these icons in personal bar and the private browsing. Thank very much for your solution anyway

Firefox doesn't store favicons for bookmarks if you are in Private Browsing mode. If you clear the cache as well then they are no longer on your computer either, so can't be displayed on the Bookmarks Toolbar. You should still see them on the location and tab bar.

I agree, it is a workaround. How is it that the firefox favicon continues to show in bookmarks when I'm in the super safe mode of browsing? Save the favicons to the same place where they won't be deleted ... like in the older, safer versions.