http://www.mozilla.com/firefox/all-older.html

Note: If anyone's reply has solved your problem, then please mark that reply as "Solved It" to the right of that reply after logging in your account. Its because so that we can concentrate on new questions.

go grab the delicious bookmarklet(s) of your choice from delicious - it's not a "plugin" but it works. after you've placed it in your browser toolbar, right-click and select properties, from there you can edit the name (because they're ridiculously long). http://www.delicious.com/help/bookmarklets

AVOS, the company that bought Delicious from Yahoo, has posted a beta version of the extension that (so far) works fine for me, and is set up to work on Firefox 4 and 5:

https://addons.mozilla.org/en-US/firefox/addon/delicious-extension/