I lost my Mozilla Firefox bookmarks when I uninstalled 2.x and installed 3.x.

It turns out they are imported automatically when you install 3.x over the top of 2.x. You are supposed to do that (I didn't know that until I read a support article - they didn't say that up front!).
If you didn't like me, you are not up a creek without a paddle: you can do from the menu: Bookmarks|Organize Bookmarks...|Import and Backup|Import HTML...
then browse to a file somewhere around:
C:\Documents and Settings\yourusername\Application Data\Mozilla\Firefox\Profiles\933n7i7y.default\bookmarks.html (something like that but not exactly)

or, instead of bookmarks.html (if you don't have it) you can pick one of your bookmark backup files. you probably have the bookmarks.html.

reference: mozilla kb article - lost bookmarks