This extension expands the built-in Password Manager with some extra features to add new items and edit currently present items and as such it should be safe. Note that you can enter wrong domain data that might expose a password to the wrong server, so, as you should always be, be careful when you deal with editing or adding new passwords manually.

Thanks for that, but I was thinking more along the lines of the integrity of the Saved Password Manager itself; is it a genuine Mozilla product or could it be a third party viral add-on that may pass my existing passwords on to its creator?

It is not made by Mozilla but anything on addons.mozilla.org is safe, and are not malware related.

https://addons.mozilla.org/en-us/firefox/addon/saved-password-editor/

You should always be careful when you install extensions, not only for security reasons, but always in respect to the stability of Firefox.

Extensions hosted on the Add-ons website have been reviewed unless otherwise stated and are usually safe to install, but they remain third-party software developed by others (only a few are from Mozilla like hot-fixes).