I've had the Flashblock and Disconnect add-ons max out my connection before which certainly wasn't the effect you're looking for with this type of add-on. Maybe try NetBalancer to see if you can find the application that's using data.
Also check out your usage prior to the spike - I previously had a similar issue, and Brumfondl correctly suggested it could be a metering issue rather than a usage one. A few people have had similar issues where the online usage meter hasn't kept track of the data usage for a few days, then suddenly adds it all at once.
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