Let’s face it — it’s not in Delicious.com’s (and Yahoo!’s) interest to make it easy to let people default to NOT sharing their bookmarks, but I wish they made it a little easier to actually do it.

So here’s how you do it in Firefox:

1. Quit out of Firefox
2. Explore deliciousBookmarks.jar (the location of this file depends on which OS you’re running) with a proper JAR editor (NOT just a text editor) and open the file yAddBookMark.xul,
3. Search for the following code:
   
<checkbox id="cb_noShare" label="&addybookmarks.sharing.label;"

    

4. Add checked=”true” to the tag, so it looks something like this:
   
<checkbox id="cb_noShare" checked="true" label="&addybookmarks.sharing.label;"

    

5. Save the file, exit, and relaunch Firefox. Voila!

Now don’t forget to uncheck it if you DO want to make something private!