Is s.yimg.com a trojan horse?

I am using Firefox 38.0.5 on a 2008 MacBook Pro running version 10.7.5 Mac OSX with processor speed 2.66 GHz Intel Core 2 Duo, 4GB 1067 MHz DDR3 Memory. I was forced to use Firefox with Yahoo instead of Safari for my work. The issue is that I think my computer functioning has been compromised. Two programs that came up on my tool bar were MAc Booster and Mackeeper out of no where. I also noticed that when I viewed information on one of the web pages I had gone to it listed s.yimg.com. I have read that this is a Trojan Horse but then another site states it is perfectly safe. Has anyone information about this. I apologize for my ignorance as I am new to support communities.