Unless you are throttling from the heat (which will cause you to lose performance while the throttling is happening), it's unlikely to cause permanent harm during the expected lifetime of the processor. It will adversely affect your electricity bill, though.
BlockTube is an extension (plugin) loaded in Firefox, and it had to have been loaded by you at some point. There may be some problem with the plugin. In any case, the purpose of the plugin is to allow you to control what stuff shows up for you on YouTube.