Comment 43 for bug 885542

It's available in nightly builds already if you're in a hurry and willing to accept some rough edges. The next non-developer version is tb24, which will be released in september - https://wiki.mozilla.org/Releases#Next_Thunderbird_Major_Release

These kind of fixes aren't backported to the stable branch (tb 17), as patches can always cause unwanted side effects nobody thought they would.