First I would go into Firefox, Options, Security, Saved Logins and in
that screen click Remove All.
The process for Thunderbird is identical
While there may be better ways, I would use a Shredder utility and shred
the Mozilla profile folders.
C:\Users\user name\AppData\Roaming\Mozilla\Firefox
C:\Users\user name\AppData\Roaming\Thunderbird
As for other data folders I would use the same tool and shred those
folders also.
I use McAfee Total protection but I think the Shredder tool is available
in other McAfee packages, and as freeware. I did use a piece of
freeware to scrub a disk on my last computer, but I forget what it was.
I found it be review the software in a search from "Shedder tools". and
"Clean old disk"
As far as cleaning the registry you some one else will need to help you.