Make sure that you do not have saved uppercase and lowercase versions of that name and password. Remove a saved password here: Tools > Options > Security: Passwords: "Saved Passwords" > "Show Passwords"

See also Password manager - Remember, delete and change saved passwords in Firefox - Use a Primary Password to protect stored logins and passwords

This is not about SAVED passwords. This happens the first time I try to fill out any field in a form.

Maybe caused by an extension.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears. You can use "Disable all add-ons" on the Safe mode start window to disable all extensions. You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

Thanks. I only have two Add-ons and I've tried the safe mode technique before. I can't believe I'm the only one with this problem. Which tells me it must be something in my settings, but darned if I can figure out what.