Pretty much what the subject says. Running the latest version of the del.icio.us Firefox extension (2.0.72) in Firefox 3 on a G5 iMac with OS/X 10.4 ends up causing excessive CPU usage. And it seems that I am not the only one affected by this.
The main symptom of excessive CPU usage is starting up Activity Monitor and watching Firefox consume 10% of the CPU, even when it is sitting idle with a single empty tab open. Disabling the Delicious Firefox extension and restarting Firefox fixes the problem.
It appears that the folks who wrote the extension don't know what's going on either, so the only "fix" is to disable that extension. For the time being, I've added http://del.icio.us/post to my bookmarks so that I can still bookmark interesting sites.