Why is backupd using up to 96% of CPU and fseventsd jumps up to 30%, at the same time? What is going on here? Have I entered an alternative universe where 96% and 30% do not add up to 100%?
If fseventsd is using lots of processor time, it must be that some process is doing lots of file open/update/close operations. (Each time any process closes a file after modifying it, fseventsd is notified, and makes this information available to other processes that want to know about changing files.)
One process that's interested in what files have changed is Time Machine. Every time fseventsd notices that a file has changed, Time Machine gets the message, and has to determine whether the file needs to get backed up. (fseventsd only tells TM which folder the changed file was in. It's up to TM to figure out which file in that folder changed. That takes time. Not normally a lot of time on each event, but if the folder has a lot of files in it and/or events are frequent it can add up. And it sounds like they're very frequent.)
From your description, the culprit is the Shockwave Flash plugin. Some website has started running something in Flash that has gone amok. Close the Firefox window (or tab) that the miscreant is running in. You'll know you've closed the right window when total CPU usage (as reported by Activity Monitor) drops dramatically. Then avoid that website.
