firefox lags constantly in many ways

Firefox hangs constantly with everything. This started about 3 weeks ago. When I type in text it lags, when I watch youtube videos it lags, and when I scroll down in websites of all different kinds it lags. The problem is getting progressively worse. It used to be barely noticeable but now it is pretty severe. This does not happen in other browsers on my computer. It kind of acts like I have a virus, but I am running it on a Mac. If I restart my computer it starts out okay, but then it slowly builds up to the same place. Firefox acts like my computer is running out of RAM but often it is the only program running.