If you don't see Import from a File… in the Firefox Lockwise menu, it means the feature is disabled. To enable it, go to about:config, find the signon.management.page.fileImport.enabled preference and toggle it to true.

See Import login and password data from a file.

Unfortunately, nothing changed when I pointed to a CSV file, Same number of old passwords

Is CSV file declared correctly, like in the article?

If you already have saved a few logins then you can create a CSV backup and compare the two files.

Thanks everyone- I'll work on this tomorrow,