I used to have the same macbook pro. The GPU's in those are notorious for failing. The lines you are getting may be because of GPU overheating. Unless you're hitting 100*C browsing the net, it can be pretty normal on that laptop, mine would always hit 90*C playing 1080p movies. And idling 50*C - 60*C was pretty common on the first gen mbp conroes.
Are the fans running at 2k rpm, and do they speed up when it gets hot? A problem I noticed with mac laptops is that depending on what program you're using, its usually a hit or miss where the fan will turn on when you hit higher temps. There's a few program that let you mess with the fan (SMCfancontrol I think its called.) For example, with VLC the fan pretty much never speeds up even though the CPU may be frying... or maybe fans just turn on too late, like 90*C.
Also, with your posts over the years I have noticed a trend in how much you like firefox. Why would you use it on OSX though? I used to use it for work (web dev plugin), but Safari and Chrome are faster by miles. Unless you have a SSD, firefox starts very slow compared to the other browsers (and scrolls like shit on OSX).
