I don't know the answer, but can suggest something to try.

Could you type or paste about:permissions into the address bar and press Enter to open the Permissions Manager.

At the top of the left column is a search box to filter the list down to the host of interest.

If you can find an entry that doesn't show a port number (e.g., it was visited on port 80 or 443), does setting Block for Store Passwords affect visits on other ports?

Note: If there is no existing entry for the host without a port number, you might be able to add it to the list by saving a cookie exception for the host.

"3-bar" menu button (or Tools menu) > Options > Privacy

If the dialog does not show "Firefox will: Use custom settings for history" select that and then click the Exceptions button. Enter the host name with no protocol or port number and click Allow, then close out of the dialogs back to the page.

You probably need to refresh the about:permissions page before that shows up.

Any difference?

Thanks for the suggestion!

I found "localhost" in the permissions. It was already set to block password storage.

I clicked "Forget about this site" Then visited again. It didn't prompt to remember the password for https://localhost, but it did prompt for https://localhost:35377. At that prompt I selected the option to never remember. I then went back into about:permissions and searched for localhost but got no results. I also get no results for localhost:35377 even though it would seem like it should be there since I chose the never remember option.

Sounds like we need a new feature to do what you want to do.