I do not use Norton security products myself. I think this article describes the latest version of the Identity Safe add-on for Firefox:

Compatibility of Norton Security Toolbar and Norton Identity Safe with Mozilla Firefox

Does that sound like the version you have, or can you update to that version? Does it work any better?

This Norton forum thread isn't very promising: Norton Identity Safe - support of local vault in Firefox 49 and up | Norton Community

By the way, Mozilla has no influence on Symantec's add-ons, they are what they are!

The preceding Norton forum thread seems to confirm that Norton doesn't plan to support a local security vault in Firefox or elsewhere and recommends use of alternative password managers. Fortunately, the Norton Identity Save still works in my current versions of IE and in Chrome, although not perfectly. (For example, the Chrome version works with most but not all log-ins, such as eBay's, which works fine in IE.)

There are many threads on this forum, but as people keep stressing it depends on what Norton decide to support and provide, and it seems Norton are moving away from supporting local storage. Of course Firefox will store passwords, and there will be many potential solutions using third party software, including Firefox add-ons.

• Password Manager - Remember, delete and edit logins and passwords in Firefox

One of the threads on this forum is

• (From 2015) How to import Norton Identity Safe Vault into FF Password Manager https://support.mozilla.org/en-US/questions/1097768

And a very long Norton Thread

• (Commenced March 2016, with a post by a Norton Community Admin) Identity Safe limited compatibility with Firefox - current status