What is the ruling on being absent from Fajr prayer in the mosque by order of one's mother due to fear of thieves, knowing that some men from the neighborhood go to and return from the mosque?

If security is prevalent in your country and thieves are rare, then your mother is not permitted to prevent you from congregational prayer in the mosque, nor are you permitted to obey her in that, because congregational prayer in the mosque is obligatory according to the more preponderant opinion. However, if the presence of thieves is widespread and it is highly probable that one is not safe from them, then this is an excuse that permits abstaining from congregational prayer. Similar to the fear of thieves in ruling is the fear for oneself or one's property from destruction or theft, or the fear of a ruler, a wild animal, or a debtor.