Typed symbols are not shown when saving draft

Steps to reproduce
User opens topic and types text in "message content" area (speed is near 4 symbols per second)

Actual result
Some symbols are not shown. When draft is saving UI "freezes" and user can't type or remove symbols

Expected result
User should be able to type/remove symbols when draft is saving

Note
It seems issue is only for Firefox (reproduced with and without AdBlock plugin), it's not reproduced in Chrome/IE