gpg --verify xyz.txt says "Good signature from ...."

Hello everybody,

I have a problem verifiing mails signed and sent using thunderbird and enigmail - here is the scenario:

1. System:
MS Windows XP SP2
Mozilla Thunderbird Version 2.0.0.9 (20071031)
Enigmail Version 0.95.6 (20080101)
gpg4win 1.1.3

2. System:
MS Windows XP SP2
Mozilla Firefox 2.0.0.11
FireGPG 0.4.8 (gmail)
gpg4win 1.1.3

3. System:
MS Windows XP SP2
MS Outlook 2003
gpg4win 1.1.3

Sending signed mails from 2. to 1. or 3. system does not make any problems.
Sending signed mails from 3. to 1. or 2. system does not make any problems.
Sending signed mails from 1. to 3. system does not make any problems.

But:
Sending signed mails from 1. to 2. system does not work correctly. When I try to verify PGP signed message, FireGPG says invalid signature (sometimes "False sign").

I already tried several settings / mail adresses, keys (RSA/DSA) and computers, but it doesn