Since Hotmail and Yahoo mail are completely separate, I'm having trouble understanding where you are having this problem. Is this problem with signing up for a Mozilla service (like Sync) or site (like this one)?

If the problem is that Firefox automatically filling in an old address, there are two different ways this could happen:

(1) Saved Login (username + password)

These are easy to remove. You can get rid of obsolete logins here:

orange Firefox button (or Tools menu) > Options > Security > "Saved Passwords" button

(2) Saved form entry

While you are typing in a field, press the down arrow to select the bad entry and press the Delete key on the keyboard.

If that doesn't work, Firefox has a feature to clear ALL form entries. To clear individual entries, you need an add-on. For example:

https://addons.mozilla.org/firefox/addon/form-history-control/

Last but not least, for the site that has the problem:

When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

orange Firefox button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

In the dialog that opens, you can open the site's cookies individually.

Then try reloading the page. Does that help?

How does that issue relate to Firefox support?