Update: When Firefox updated to 4, my TweetDeck Twitter app also stopped working. After I uninstalled 4 & reinstalled 3.6.16, TweetDeck started working again. I don't know if this is coincidence or not, but I'm not upgrading to 4 again until 3.6 is no longer usable.

I found instructions for reinstalling Firefox 3.6 here: http://support.mozilla.com/en-US/questions/793360#answer-147853

I found this in a forum somewhere and it works...

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.pdf 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.

The above fix works for Windows too... I found my "install.rdf" file here: C:\Users\[USERNAME]\AppData\Roaming\Mozilla\Firefox\Profiles\1nc1d3r.default\extensions\{2fa4ed95-0317-4c6a-a74c-5f3e3912c1f9}

Probably should have specified: Windows 7

I was attempting to do what you suggested above and everything was going great until I got to "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." I don't have a em:maxVersion+4.0b3pre". I have em:marVersion="3.7a1pre"! Do I just need to change mine to reflect the 4.0 or what? Any help you can provide will be greatly appreciated.

This was pretty simple to add back in. I put the Delicious address in the address box, right clicked on the Delicious icon displayed in the address bar, a menu came up, I chose Bookmarks toolbar. I now have the Delicious toolbar to access my bookmarks there. I'm using Win 7 64 bit.

This problem's still present in FF6 and 7 - on my work machine, running FF6, I found I could install the Delicious 2.3.1 beta from https://addons.mozilla.org/en-US/firefox/addon/delicious-extension/ and it worked fine. However, without any xml hacks or workarounds, the same beta plugin doesn't work in Firefox 7 on my home machine.

Sigh.