The group permission Edit Time Limit might be useful for some boards, probably as a moderation mechanism. Unfortunately, this permission doesn't distinguish between threads (first posts) and replies (posts), and if set to a value higher than zero, this will block edition of all posts alike. To change this, you can import the attached file to the Hooks plugin system.

This should accomplish the following:

• The edit button for threads (first posts) should be visible, based on permissions.
  
• Users should be able to always edit threads (first post), by quick or full edit, based on permissions.
  

In summary, this should completely ignore the Edit Time Limit permission for threads (first posts), respecting all other checks MyBB does for the user or replies.