Wow, that took multiple minutes to run here.

Well done to the author of this hack on finding a way to dump massive amounts of data to firefox's sqlite store.

Not tackling the same problem but in the spirit of removing things and Firefox, you can bring up a url in the address bar, press down arrow, press Delete, and it'll go away

It only deletes that very specific url though, like foo.com, however foo.com/bar will still exist

I think that only works before Sync replicates all those crufts to every other device.