Using your editor or Word Processor of choice, compose your Signature and save it somewhere on your Local PC as type ASCII or TEXT
Note: if you do not save the file as text it will be useless as a signature.
Tip: If you use iFolder, and save your Signature file to your iFolder you can use the same file from work and home.
From Mozilla Messenger go to Edit -> Mail & Newsgroup Preferences

Highlight your email account name (FACS, imap.fcs.uga.edu, etc.) and check the Attach this signature: box, and click on the Choose button to find the file you saved earlier.

Click OK and your Signature file will now be attached to your outgoing messages. You may need to close and re-open Mozilla to activate the new setting.