Thunderbird version 18.0b1

New:
- PNG avatars
- Setting to display ISO date/time in message list
- Fixed-width font for composing

Changed:
- Attachment summary moved to header for visibility

Fixed:
- "Delete (from notification)" setting did not retain state
- Find folder search hint text was cut off
- User not notified if offline when sending message
- Warned "Sent folder not found" when uploading sent messages was disabled
- CC/BCC fields were not expanded by default for Reply All
