It's not related to Firefox. It's probably a feature provided by McAfee to protect your passwords used on the web. You can disable that in the web protection settings of the AV. Check the McAfee support forum/FAQ.

It may be worth noting Firefox will itself save passwords and if required Sync them between other devices running Firefox.

• Password Manager - Remember, delete and edit logins and passwords in Firefox
  • Use a Primary Password to protect stored logins and passwords
• How do I set up Sync on my computer?
• There are also quite a few password related addons search link

Although outside of the cope of this forum it may be worth remembering other software may be able to store passwords.

• https://wikipedia.org/wiki/List_of_password_managers

Users of some of Norton|Symantec products are currently having issues with an optional free password etc storing utility not working with Firefox for the best part of a year.

• see /questions/1097768 How to import Norton Identity Safe Vault into FF Password Manager

Mbduke said

... says "Following browsers were found, but not supported in Password Bank: Too new browsers: Firefox 45.0.1 (x86 en-US) How does this relate to Firefox? Is there something I need to do?

Sounds like McAfee is a bit behind in updating their Firefox add-on.

IMO, most security suite add-ons for Firefox create more aggravation for their users than they are worth in the long run. Personally, I specifically avoid security suites which 'insist' upon installing add-ons for Firefox; few of those 'suites' manage to update their add-ons in a timely manner and when an add-on password manager "takes over" for Firefox saving website passwords the user is SOL for entering that data when the add-on loses compatibility with Firefox after a Firefox update. IOW, too many times it seems as if those companies are surprised when Mozilla updates Firefox - like it is a secret that a new version is released. If they are adding onto Firefox it is their duty to provide timely updates for their add-ons; which few seem to do based upon what I see here in this fora.

https://wiki.mozilla.org/RapidRelease/Calendar Mozilla provides a calendar of the release dates for the next year; 4 channels of 6 weeks each, with a few exceptions at the end of the year. And anyone can test the pre-release versions to test the compatibility of their add-ons prior to the actual Release of each new Firefox version.

Currently, Norton / Symantec seems to be the only security suite software company that manages to 'get it right' by having their new or updated add-on's for Firefox available for their customers within hours of Mozilla releasing a new version of Firefox. But sadly they aren't perfect; around Firefox 40 their Firefox add-ons weren't fixed for a month or two due to them not catching a major security change or two that was made in Firefox and it took them a few months to get things straightened out. Hence the reason that I avoid security applications that insist upon "adding onto Firefox; "do I avoid updating Firefox because a 'silly little addon' would quit working". And for the most part those add-ons for Firefox do little to provide additional security; most merely 'extend' features that are built into Firefox already or merely provide an alternative source for anti-phishing or anti-malware data to be uploaded from to the device.