How Can I Get Firefox To Retain Correct Default Values In Entry Boxes?

It used to be that whenever I returned to a site that I used a lot that had an entry box for say my zip code, I only had to enter the value once and it would automatically be entered as a default value whenever I returned to the site. This feature stopped working for me sometime ago. I’ve scanned all the Firefox (3.6.3) options and can’t find anything that will help me configure this. Any ideas on how to fix this would be appreciated.
To make matters worse, old values that are no longer valid keep being entered as defaults and I can’t change them. E.g. the location box at http://movies.yahoo.com/showtimes-tickets/ always defaults to ‘Los Angeles’ even though I live nowhere near there and have re-entered the correct zip code many times.