Rated 4 out of 5 stars
It is TB 60 compatible!
You just need to rename startupmaster-1.6.5-fx+tb.xpi file to startupmaster-1.6.5-fx+tb.zip.
Then you need to open it with a zip manager (I use 7zip).
It will be composed of different files and subdir.
Identify install.rdf file (in the root dir). Open it with an editor like notepad because the extension .rdf is really a .txt extension.
Identify the Thunderbird section of the file and change this line:
Save the file and repackage StartupMaster with a zip manager, You should get a startupMaste.zip file.
Change again its extension from .zip to .xpi and now install it.
This way it works also with TB 60!