As annoying as undoing all my settings on passwords (no don't save those, thank you very much), I realized, belatedly, that I was having to stop videos constantly. In the course of turning off autorun (no setting in firefox for this -- requires a plugin; I feel like the web is sort of like landlines. Not Keeping Up with the Standard Set By Mobile), I realized that adblock had also gone away in the update.
Wonder what else I'm going to remember in the next day or so. Good news -- adblock and flashstopper seem to have fixed the super sluggishness I've been annoyed by post upgrade.