I cannot confirm this problem, but to be honest, I haven't been doing much copying/emailing of my RDF file either.
I do on a regular basis copy one particular RDF file that has the account settings for our domain to a users computer, and have never had a problem with it generating wrong passwords, but it isn't a very good test case either...
John, could you post your list of installed extensions and plugins?
Also, I'm a little confused on the problem description though...
What did you mean by:
only to find that when I try and log into the same sites as on the first PC the wrong passwords would be inserted into the password field only too be replaced by a automatically generated one
and then
I have Firefox password manager turned off
If a password is being entered BEFORE Passwordmaker is entering one, then something else is populating your password field.
Are you quite certain that you don't have some other password manager in play?
If not, then one thing I can suggest is to do a checksum on the original RDF file, and the one that ends up on the second computer, and see if the file is actually the exact same file, bytewise.