Now I am confused. You say that this problem has been plaguing you
since the very first version of SeaMonkey 2.0 (that is before SM 2.0.1).
I did see this problem more than a year ago, but I did not see this
problem in SeaMonkey 2.0.14.
I've said I'm experiencing the problems described in Bugs #354706
and #355063 ever since SeaMonkey 2.0. I haven't really got a chance
to experience the problem you describe in Bug #348941, previously,
but you may try the attached sample with SM 2.1:
* Save both files in a same directory and open "username.html";
* Enter whatever name and "Continue" - you'll be navigated to the
"password.html" page;
* Enter a sample password and "Log in" - you should be given the
option to save that password;
* You've been just navigated back to the "Username" page and you
could either enter a new name, or start typing previously entered
one to get an auto-complete option (alternatively double click in
the text field);
* When you "Continue" once again you should have the stored
password prefilled on the "Password" page;
* If you enter a new password you should be given an option to
update the saved password with the new one (only one password
associated with no username could be saved for a site).
Such local test appears to work with SM 2.1 but one may need to
serve the pages off a web server in order to verify the behavior
with other browsers.