I haven't been able to clearly understand just where my passwords are located, so does anyone have any suggestions on where to find my passwords?  I'm stuck....

I recently had to update all my passwords for all my Thunderbird email accounts. It all seemed to go well. I was able to logon to all my email accounts, but just 2 days ago I wasn't able to logon to one of my secondary email accounts. I haven't been able to clearly understand just where my passwords are located, so does anyone have any suggestions on where to find my passwords? I'm stuck....