I can reproduce the issue on the Smartermail build for Linux from last week, and a version of Smartermail from several years ago on a Windows server.
Do you have autocomplete turned on in Firefox?
If you mean auto-fill (the built-in password manager), I do on one version of Firefox and not on another. In both versions the behavior is the same. However on the version with the password manager, I can at least login if I click it from the dropdown because it fills both fields simultaneously.
are you using any additional ad ons for FireFox ?
Yes. I tried on a fresh version of Firefox Developer Edition with no configuration changes and the login prompt works as expected.
If I turn off the Bitwarden Extension in the versions of Firefox I observed this behavior, the login prompt works as expected too. It seems Bitwarden causes this bug to occur when it modifies the Javascript on the page.
Edit: I can also reproduce this bug in Chromium-based browsers when the Bitwarden extension is installed and the vault is open.