From MozillaZine Knowledge Base
How can I use multiple From: addresses in Mozilla Mail?
Mozilla version 1.7 includes Multiple Identity Support. Unfortunately, it's for advanced users and requires manually editing Mozilla's configuration files.
Another option is to create a 'fake' mail or newsgroup account for each alias email address. After you create it, change these settings under,
Edit >> Mail & Newsgroups Acccount Settings >> [account name]:
- Set the fake account to never check for mail: Under Server Settings, uncheck all the boxes.
- Set the fake account to save Sent messages, Drafts and Templates in the same folder as your 'real' e-mail account: Under Copies & Folders, change the locations.
A third option is to set a Reply-to address using your alias address. Your e-mails will still look like they're From your real address, but when someone clicks Reply, the message will be addressed to the Reply-to address: Simply type any Reply-to Address you want under,
Edit >> Mail & Newsgroups Acccount Settings >> [account name]