For those of you who are really missing your Delicious bookmarks after upgrading to Firefox 4 (or who haven’t upgraded because there is no Delicious add-on yet), there is a solution! First, go to the Delicious add-on page, click the “Continue to Download” button. Then, instead of simply clicking the “Accept and Install” button, right click it and save the resulting XPI file. For all practical purposes, an XPI file is simple a ZIP folder for Firefox add-ons. You’ll need to open that folder and make a few changes. First, open the install.rdf file with a text editor and find this line:
This is what is keeping the add-on from installing properly. To allow the add-on to be installed on any version of Firefox 4.0, you’ll need to use this line instead:
Make sure you save the the changes you’ve made. Next, delete the entire “META-INF” folder from within the XPI folder. It holds the checksums of the original add-on. Now that we’ve modified the install.rdf file, the checksums are wrong. Removing the “META-INF” folder will take care of that. Now open Firefox and go to “File > Open File… ” Then find your newly modified Delicious XPI file and open it. This should bring up the dialog box for installing Firefox add-ons. After restarting your browser, you should be greeted by a fully functioning Delicious add-on running on Firefox 4!
Update: It has come to my attention that there is also a beta version of the plugin with Firefox 4 support: https://addons.mozilla.org/en-US/firefox/addon/delicious-extension/