Seems like this might be an active issue on Firefox (and possibly other browsers). See here for some discussion on Reddit: [https://www.reddit.com/r/firefox/comments/t8ap7n/firefox_wont_fill_in_username_for_twitter/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button Firefox won't fill in username for Twitter] And the bug report here: [https://bugzilla.mozilla.org/show_bug.cgi?id=1733944 No username autofill or manual fill in Twitter login page]

What do you see if you right-click in that username/email field? Is there a "Use Saved Login/Password" in the context menu or possibly "Manage Logins" ? *https://support.mozilla.org/en-US/kb/password-manager-remember-delete-edit-logins