The Delicious extension for Firefox doesn't seem to have been updated for Firefox 4.0 yet, but here is a fix that can be done to make it compatible with Firefox 4.0/

From this thread:
http://domino.symetrikdesign.com/2011/03/25/getting-the-delicious-bookmarks-add-on-to-work-with-firefox-4-0/comment-page-1/#comment-5807

I figured out a simple trick to make the Delicious bookmarks Firefox add-on work after upgrading to Firefox 4.0.

This assumes that you previously had the Delicious bookmarks add-on working in Firefox 3.6.x and you have just upgraded to Firefox 4.0 – which disables the add-on and states that it is incompatible with Firefox 4.0

• Open Firefox
• Goto “Help” in the Menu, then “Troubleshooting Information”
• Click on the “Open Containing Folder” button (within the box “Application Basics” on top of the page)
• Close Firefox
• Goto into directory “extensions\{2fa4ed95-0317-4c6a-a74c-5f3e3912c1f9}”
• Edit install.rdf and change the following from the first line to the second line

     em:maxVersion="4.0b3pre" />
     em:maxVersion="4.0.*" />

• Restart Firefox

Worked like a charm--just a clarification for those who don't normally do that sort of thing: you have to find the line in the file that looks like the first line in the note--then make it look like the second line.

Thanks for the work around tip! Works like a charm indeed!

Thank you so much the-edmeister! That did work like a charm :) You're awesome!