I get only a time display for bookmarks that were created the same day. If all of your bookmarks are showing only time, even those you know were created a long time ago, it may be because they were "imported" or otherwise recreated during the upgrade process and therefore are "new" as far as the date stamps are concerned. I'll play around with importing some old bookmarks and see what happens...
Something you can try:
The default storage location for the Bookmarks.html file (all your bookmarks) is C:\Documents and Settings\[UserName]\Application Data\Mozilla\Firefox\Profiles\[RandomChars].default (see
Mozilla Knowledge Base Article on Profiles). Note this is a hidden folder. Under this folder is a bookmarkbackups folder which may contain prior backups of your bookmarks from the previous Firefox version. You can view these files with a text editor and determine which are in HTML format and from the earlier Firefox version. You can then take one of these backup files, change the extension to .html and import it into your current Bookmarks file from within the Organize Bookmarks tool in Firefox 3. I recommend you make a backup of your current bookmarks first from the same Organize Bookmarks tool.
Update: Any bookmarks (IE) I import have a date and time stamp of when I imported them and only show up with a time (until tomorrow). So the above may not retain the original datestamps. I don't have any previous Firefox bookmark files to test.
You can start including date info in the bookmark itself so it survives future imports/browser changes. What is the date on your bookmarks looking like today?