MacBook not performing well - slow on YouTube, Firefox etc

Hey..

I bought this second hand (but great condition, looked after etc) and since I've had it I've noticed a couple of things which are really starting to bug me.

In embedded video like YouTube and other video-based websites, the video will pause for a couple of seconds now and again - I've also noticed this in animated gifs too now.

Also, in Firefox, scrolling with the up and down keys sometimes takes a couple of seconds before it moves, same happens with two fingers scrolling on the track-pad and mouse roll-wheel.

Now, whilst typing this, sometimes the text won't show up immediately and will appear really quickly (as if I'm typing a lot faster) briefly.

Any idea what's going on here?

Thanks!

 

It happens every day that i use it (shutting down and booting up each day).

How do I get to the activity monitor?

Edit: Cmd + Space for the win

 

Jeez, totally forgot - could be a rogue Firefox add-on or two.. I just deleted a few, and it FEELS faster, whether it really is or not I don't know but I'm testing it out now by simply typing into this here box, and so far, no slow down from inputting text, and scrolling down windows seems faster - the true test shall come from a YouTube video or something.

Cheers dude, as Firefox was at close to 100% on the activity monitor!