Rated 5 out of 5 stars

by addon yahoo on Aug. 6, 2014 ·

Even the grey bar has gone now.

You only have to make two small modifications, because Thunderbird thinks it is not compatible any more.

Rename {2af6ce1e-fbf7-11db-8314-0800200c9a66}.xpi to {2af6ce1e-fbf7-11db-8314-0800200c9a66}.xpi.zip
Open it and open the file install.rdf with a text editor.
In this line: <em:maxVersion>6.*</em:maxVersion> change 6.* to 99.*
Save the changes and replace the old install.rdf in the zip-file.
Change the name of .zip back to .xpi
Put the changed .xpi in the folder %APPDATA%\Thunderbird\Profiles\default.YOUR-PROFILE\extensions

Go to Tools -> Options, select the Advanced panel, select the General tab, and click Config Editor. Click "I'll be careful, I promise!" to proceed.
Change "mail.tabs.drawInTitlebar" to false.