That question is not about Firefox.

If the way the mail is sent is thru your e-mail provider, there may be a setting that adds those you send mail to get added to your address book. Check with your provider for more information.

Here's a little detail about the problem. I am a real estate broker. When I log into the Multiple Listing Service (MLS) with Internet Explorer, after I send an email to a client, whenever I send one thereafter, the email address auto populates for me. When I do the exact same functions after logging into the MLS on Mozilla, it will remember addresses for that session but when I log in the next day, there are no email addresses saved. I am using the same email provider in both cases....I think! Joe

You are using two different browsers, meaning there would likely be two different settings. Check them carefully on each browser.