Go through the following steps:

  1. Get a ZIP-compatible archiver (such as 7-Zip)
  2. Get the delicious addon (go to the download page and then right-click to download rather than install)
  3. Extract the .xpi with the ZIP archiver
  4. With a text editor edit install.rdf and replace em:maxVersion="4.0b3pre" with a version greater or equal to the current Firefox version (i.e. em:maxVersion="4.0")
  5. Remove the META-INF folder as it contains signing information which gets broken by messing with the content as we did in the previous step
  6. Recreate the .xpi by zipping the files previously unzipped, and then change the .zip extension to .xpi
  7. Drag the xpi file and drop it on an empty Firefox tab to install it.

Delicious will eventually support Firefox 4.