I have a 13" Macbook Pro Retina, running High Sierra. I also experienced this problem when running the previous os version. It started happening right after getting Firefox 57. When I open a few links in new tabs, my CPU usage goes way up, the fan starts going on overdrive, my macbook turns into a space heater, and my batter goes down by a couple %. Prior to 57, I never had this issue. Didn't matter how many tabs I opened, the only thing that caused my cpu to sweat was heavy flash usage. I have adblock, privacy badger, disconnect, and flashblock, and I have the autoplay disabled in the about:config. I have tried starting firefox with no add ons enabled, but with no change. Is this normal now for opening many links in new tabs, or is something wrong? Thanks.