There is a fix which I found at this url: http://forums.macresource.com/read.php?1,1125540,1125571 It links to another site so I've copied the quote from that site:

   From the menus select Help > Troubleshooting information
   On this new tab, near the top, you’ll see a field called Profile Directory. Click the button in the field to launch Finder (File Explorer for Windows).
   In the window which has popped up navigate to the extensions folder. Inside that should be another folder named “{2fa4ed95-0317-4c6a-a74c-5f3e3912c1f9}”. Inside this is a file called install.rdf
   Open install.rdf in any plain text editor.
   Around line 8 you should see some XML which looks like this: em:maxVersion="4.0b3pre" />
   Update this to em:maxVersion="4.0" /> (See we removed the b3pre bit?)
   Save the file and restart Firefox.
   Boom. The delicious extension will now work.

thanks!!!I was crazy to make it work...

Thanks for this quick fix!! I was ready to find a new browser before giving up the easiest, most efficient bookmarking tool I've ever used. Hopefully, any one else using delicious bookmarks (del.icio.us) on Firefox will find this site too!

thanks. this works. one more tip - while you're doing it, change em:maxVersion="4.0" to "5.0" - it might save having to redo it next time!

WARNING: there is a compatibility issue with FF 4.0 and the awesome bar integration of this extension, if enabled ALL ADDRESSES disappear from result list!

Here a screenshot: http://i51.tinypic.com/2anqqq.png

So, in the Preferences of the Delicious extension, you should disable the "Awesome Bar Integration".

Hoping that someone fix this issue...

Thanks for info, l followed the steps and made changes as you suggested but it has not fixed this problem. Can you suggest anything else?

You need to set that to 4.0.* so that it is compatible with all 4.0.# versions. 4.0 limits it to only the 4.0 version.