Comment 54 for bug 885525

(In reply to ncnmra from comment #47)
> A workaround I found is:
> 1) disable "Show In IMAP" under Settings>Labels in the Gmail web interface
> for the Gmail Trash folder
> 2) unsubscribe from the Gmail Trash folder in TB

Logic on trash in DiscoveryDone() was one like following;
      <= At this stage, trash in XLIST is perhaps already set as SpecialUse=Trash
         (perhaps only when IMAP delete model = "Move to trash" model)
  Search SpecialUse=Trash folders.
  If(number of SpecialUse=Trash folder>1) // Keep single SpecialUse=Trash folder
    For each SpecialUse=Trash folder
     If(Gmail){ if trash in XLIST, setflag(Trash); else clearflag(Trash); }
     Else{if folder of trash_folder_name, setflag(Trash); else clearflag(Trash);}
By your workaround, number of SpecialUse=Trash folder is changed to 1.
I think it's reason why your workaround works.