Networx is great for monitoring overall quotas though you can install NetBalancer (free version)
which will enable you to scrutinize individual applications. The free version allows you to set up to 3 application restrictions (you can swap these as you go along) and so drill down on how much bandwidth individual applications are using.
HowToGeek has a detailed tutorial
about installing and getting the best from NetBalancer, which I recommend you to follow.
If you are using Firefox I suggest you download Chrome and monitor what happens (with NetBalancer) when you perform very similar browsing sessions. Say an hour with each browser spending the same amount of time surfing the same sites, at different intervals of the day for both.
I've seen this happen with Firefox though mainly when surfing video sites like YouTube. When you only part watch a video, and then jump to another, and so on.