1. Type about:config in the address bar and press enter
2. Accept the warning
3. Type gecko.handlerService.schemes.mailto in the search box. You should get 8 lines.

Do you see sbccglobal.com in one case of the value's column? If yes, right-click on it and select Reset.

You lost me at #1. Typing about:configure in the address bar doesn't get me any where because it's not a complete web address.

Sorry… about:config is a specific page in Firefox in order to access to the advanced configuration. There is no other way to open it.

See this article for more information: Configuration Editor for Firefox

Firefox has a lot of built-in special pages that use the about protocol to access them. You can see all of them on the about:about page.

This is a very strange problem. You are loading the exact same inbox in both accounts, but when you reply in Safari your message lists a different reply-to address than when you reply in Firefox?

Can you confirm that the address bars of both browsers list the same address for the inbox? Sometimes if you use an old bookmark for Yahoo mail there is a glitch in redirecting you to the latest server and bizarre things happen. Try this general address (which Yahoo should redirect to the latest server for your account) to see whether it helps: https://mail.yahoo.com/ or if you use AT&T Yahoo, use the mail link here: https://att.yahoo.com/

Thanks for your responses. I solved the problem by creating a "Add send-only address" in my settings, using my original email address.

Again, I appreciate your attempts to help!

I'm happy that you manage to fix your problem. Thanks for your feedback.

You can mark your last reply as chosen solution, it could maybe later help other users with the same issue.