There is a bug holding up enabling import from a CSV file, but you may still be able to use it by making a settings change. I have not tested this myself recently.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste signon.management.page.fileImport.enabled and pause while the list is filtered

(3) Double-click the preference (or click the toggle button at the right end of the row) to switch the value from false to true

The next you open or re-load (Ctrl+R) the Logins & Passwords page, a new item should appear on the ••• menu.

Does it work?

Thanks for the quickly reply. I actually just found out that we are now using LastPass and are not to store these passwords on the browser, so it looks like I will not be trying this after all. Sorry I can't confirm if this works! Maybe it will help the next person and they can let you know.