Rather than saving the user information in the script file, something like this can be used for all the variables.
if (!GM_getValue('meeboUserName'))
{
meeboUserName = prompt("What is your Meebo user name ?? ");
GM_setValue('meeboUserName', meeboUserName);
}
else
{
meeboUserName = GM_getValue('meeboUserName');
}

This script asks for the variable value once, if not found, and then saves it for later use.

P.S. : You can see you password at about:config, filtering by greasemonkey, so this is not really safe either...

Stefans mods work fine. The first part your replacing is just adding an Asterick to whats all ready their. The last several "document.getelementbyid"'s are each a separate line that replaces the similar listing. Document is the beginning of each line.

This script no longer works with the final current release of meebo.

You can fix it by replacing two particular chunks with this chunk:

// @include http://www*.meebo.com/*
// @include https://www*.meebo.com/*

and this chunk:

//Don't edit below here
document.getElementById('aimid').value = aimID;
document.getElementById('aimpassword').value = aimPwd;
document.getElementById('yahooid').value = yahID;
document.getElementById('yahoopassword').value = yahPwd;
document.getElementById('msnid').value = msnID;
document.getElementById('msnpassword').value = msnPwd;
document.getElementById('gtalkid').value = jabID;
document.getElementById('gtalkpassword').value = jabPwd;
document.getElementById('meeboid').value = meeboID;
document.getElementById('meebopassword').value = meeboPwd;

I do like it a lot myself and so far (in my roughly 15 minutes of search) it's been the only best solution I can find after the extension I was using seems to have stopped always remembering passwords. If there was someone slightly more knowledgeable and knew how to write a script that replaced the autocomplete = "off" statements in the html I think that would be a much better solution (as then the password can be saved in the normal firefox password list and be more secure.) The script would then simply have to autosubmit the form when it found a a password was entered.