This is the package signature as generated by The GnuPG. This signature identifies the party that created the package. By verifying it you are sure that the package coming from a trustworty person or organization.
Here is the package signature:
-----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) iEYEABECAAYFAkwhOl0ACgkQqesEzEdeXBFjKACfRqgL4gcZkL+ulc8aDZxOjygq giYAn3f0OZoxcOLiETzbzlKmu2haK5Un =dKRB -----END PGP SIGNATURE-----
You can save this signature in a text file (e.g. thunderbird-3.0.5bg-i686-1gds.txz.asc) in the same directory where the package file (thunderbird-3.0.5bg-i686-1gds.txz) resides and you can verify it by issuing the following command:
gpg --verify thunderbird-3.0.5bg-i686-1gds.txz.asc
NOTE: You need to import the public key of SlackPack first:
gpg --import KEY