I am running Firefox 38.0.1 on my Mac, which is using Yosemite 10.10.3. I am also using SMC Fan Control. When I run Firefox, the temperature on my Mac shoots up from about 43C to around 78C and hovers there, causing my fans to revs way up. I am not running video or ANY other programs in the background. I have exactly 2 websites open now, Yahoo.com and this Firefox help page. The only other program running is Astrill VPN. When I close Firefox and use Safari to run the exact same websites, plus about 9 others, the problem disappears and my macbook pro runs at a steady mid to lower 40s Celsius. This began when I updated to the newest version of Firefox. No other software was installed prior to that for over a month and since the problem began, only an update to my adobe flash player has been installed. I don't like Safari. I much prefer to use Firefox, but not with the system rapidly heating up as it is. You guys really need to look into this.