It took me forever to find anything on this with google, so I'm blogging it in the hope of making it easier for the next person to find. It turns out the Mozilla Knowledge Base has a terrific article called Bookmarks not saved that discusses lots of ways this can happen. Go through the list and you'll probably find a solution.
In my case it was the second item: "Places preferences - Firefox 3". It turned out that all three of my boolean browser.places preferences were set to non-default values -- not by me, since I'd never heard of any of the browser.places preferences before. After toggling all three of them by double-clicking, Firefox lost its anterograde amnesia and started remembering new bookmarks again.
[ 23:01 Mar 18, 2009 More tech/web | permalink to this entry | ]