Bludshot, I'm glad you posted this. The same thing happened to me a week or two ago; I opened Firefox on my Mac only to have a message the Block site had been installed, but neither my husband nor I had installed it, and I'm sure it's not one I picked up in the past. I had read about it, probably a year or more ago, but never thought it would be useful enough to want it on my computer. I removed it and forgot about it until tonight.
Tonight, the same thing happened with a different plugin, DuckDuck Go. It happened while the computer was in sleep mode, and when I went to "wake" it Firefox was sluggish and freezing up.
I don't know what's causing this, but I'm going to run a malware check and also make sure I'm not running something that allows for a hidden Java script to start wreaking havoc.
It's weird; weirder still that this seems to be the only place I can find anybody mentioning this happening.