Did Firefox "auto fill" the field with a saved login? That can change the appearance by applying a color filter (yellow fill). But yours isn't yellow, and I don't know what the little green icon is.

A green letter-shaped icon indicates that this field is for entering an email address.

The browser recognizes the email input field in the form on the page and automatically replaces it with its own.

Double-clicking the left mouse button on this new field opens a list of recent mailing addresses from which to choose.

How to disable autocorrect in browser settings?

Can you provide a working example of this issue on jsfiddle.net or codepen.io?

It's not about the code.

Only in the Mozilla browser there is an auto-change of the email address input field.

I suppose this can be fixed in the browser settings. How?

It's orange for me. But this green icon on your screenshot looks suspicious, as jscher2000 stated before. Don't you have any addon that operates on input fields? How does it look like in the Safe mode?

A good idea! It was a Mozilla mailbox add-on. Thank you very much!