This just a guess. Ok?
If it's like Firefox: Tools > Options > Privacy > Saved Form Information
Then put a check in the box: Save information I enter in Web page
forms...
The preferences set up in Mozilla is somewhat different than FireBird /
FireFox.
I don't allow any of my browsers to auto-fill forms (just a personal
thing), but I checked my Mozlla 1.6, and I think this is what you are
looking for:
Edit -> Preferences->Privacy & Security->Forms->
Then there is a a Button on that screen called "Manage Sites" which
displays a pop-up dialogue with two tabs: "Forms Never Previewed" and
"Forms Never Saved". The latter tab keeps a listing of sites for which the
Form Manager "will never ask to save forms...". I'm guessing that when you
accidentally checked the box "not to see this popup again", it added the
site to this list. Simply remove the site or sites.
If I misunderstood and the suggestion doesn't work, the option you are
looking for should still be somewhere in the general vicinity of:
Edit -> Preferences->Privacy & Security->Forms->