Saved passwords and usernames not recognised

[#1]

This problem exists on links that haven't been chnged for years. Have tested about a dozen assorted solutions but nothing found to solve problem. Believe it may be connected to security, still investigating. I was using Duck, Duck Go. until I discovered it was the cause of computer speed deterioration. Now Using Firefox, (Free)AVG and Windows 10. Thanks for any possible solutions other than to upgrade to Windows 11?

[#2]

Moderator's note:
Thread moved from Broadband to Tech Help

[#3]

a dozen assorted solutions 

1) try the credentials in different browsers

2) try the password reset option

3) try to open a new a/c