how to limit network usage to only the web browser (Safari or Firefox)

I sometimes use a 3g network (router is a google nexus phone over bluetooth if you must know).

It works fine, except Mac Os and lots of the applications do network traffic during that time that eat my monthly data cap in a few hours (software update, mail, etc ...).

What I'd like is to easily go into a mode where only my web browser (Safari or Firefox) can access the Internet.

And then when I'm back on Wifi, I'd like to enable the default behaviour.

I tried the Apple firewal, but even if I disable incoming traffic, those apps still generate network traffic, and I want to disable all traffic except my web browser temporarily.

Thanks,

-- Georges