How do you stop pre-typed text from showing up in the Username field? Anyone know?

I'm on Linux version of Firefox. I went to a website, and I typed the first letter of my password to log in, and at that point, a drop down list appears with my various usernames and e-mails that belong to other sites, out of the three that appeared.

I tried to Google the answer, and apparently the "solution" is to press the "delete" key in Windows, and "Shift Delete" on a Mac. Well I tried both, none worked I presume that's because this is all happening on a Linux system.

Any help would be appreciated...

Pen guin