Freezing breast milk in glass mason jars is considered safe, as long as the jars are properly sterilized and you leave enough space at the top to account for expansion when freezing; glass is generally considered a good material for storing breast milk and is a safe option for freezer storage when used correctly. 

Key points to remember:

• Use freezer-safe jars:
  Ensure your mason jars are labeled as freezer-safe. 
• Leave space at the top:
  Always leave about an inch of space at the top of the jar to accommodate milk expansion when frozen. 
• Sterilization is crucial:
  Before first use, sterilize your jars by boiling them in water for at least 20 minutes.
  
• Wide-mouth jars preferred:
  Opt for wide-mouth mason jars as they are less likely to crack when freezing.