Firefox had marked the bookmarks file I'd copied from the most recent Mozilla "hidden", and had gone ahead and made numbered bookmark files up to bookmarks-6817.html! No wonder I was low on disk space, they took up a Gig and a quarter.
I made a new install of Firefox and it did the same thing. Marked bookmarks hidden and started making numbered ones. I unhid it and copied the highest numbered one to bookmarks.html and this activity seems to have stopped, as has the 100% CPU problem.
Weird enough.