See

• http://kb.mozillazine.org/Password_Manager#Exceptions
  It is a long article and probably worth scrolling through.
• and Password manager - Remember, delete, change and import saved passwords in Firefox
  • Usernames and passwords are not saved_password-manager-settings

With luck all you need to do is find and delete the exception that you have made. But other possibilities include: that you are using private browsing or the site is preventing saving of usernames.

Some web sites, including many banks and a few other sites, tell browsers not to remember passwords for visitors by using autocomplete="off".
Which is what that page does, Firefox respects the web page author's coding in pages like that.

This bookmarklet makes the browser ignore autocomplete="off" temporarily, allowing you to save passwords even on sites that use autocomplete="off".

Remember Password bookmarklet:
https://www.squarefree.com/bookmarklets/forms.html#remember_password

Or, you can use the Saved Passwords Editor extension for web sites where Firefox doesn't offer to save the Username /Password.
https://addons.mozilla.org/en-US/firefox/addon/60265/

Thanks the extension worked well

You may also use the "Bookmarklet" to reset your Password Saving Options. (EX re-force/"Force" Firefox to save Passwords

Found Here: http://kb.mozillazine.org/User_name_and_password_not_remembered

Scroll down the page to "Bookmarklet", follow the instructions.

.......................... Also, remember some websites by design will not allow saving of a Password (common with banking,etc.) It's not germane to this thread, but you can look up on the web "URL Hacking" (and save password) or some combination thereof. The word "Hacking" is FAR from "True"---all you're doing is changing a "false" to "true" or "true to false" or 0 to 1,etc. That may or may not work if you REALLY want to save a PW for the site. (however, like Banking,etc.), probably best not to keep saved. Your call hope the fix above works.