You would probably have use the Saved Password Editor extension if you had a toolbar button.

You can open a standalone Password Manager window via "Tools -> Page Info -> Security", also accessible via the Control Center 'i' icon icon (More Information).

Would it be possible to create an addon that added a toolbar icon that opened the standalone Password Manager window? I am a programmer but have never looked at Firefox addons. If it is possible I'll try to figure it out.

Thanks.

The API for the Password Manager isn't complete, and I saw posted yesterday that the new API wouldn't be ready for Firefox 58. So it looks like add-ons for modifying or appending the Password Manager feature might not be available any sooner than Firefox 59.