There are free add-ons for Firefox, and yes, that might help reduce the amount of data.
If there are a number of PCs in a single home it might also reduce traffic if 'Flash' was uninstalled. It may still be needed on one PC, just so you can see certain websites, however.
There are some other ways to reduce/remove adverts, but get more technical. On most PCs there is a file called the 'hosts' file which can be used to force a PC to use a particular IP address for a website, instead of doing a DNS lookup.
When you try to connect to some website - for example www.bbc.co.uk - there is a "lookup" (like a directory enquiry call) to find the IP address (in this case the BBC is 212.58.244.142) for the service you want.
In most cases the DNS lookup is done via your ISP, but it is possible to override that by having entries in your PC's 'hosts' file.
On Windows it is hidden deep down with 'internal' parts of Windows (on an XP machine, for example it may be found as C:\Windows\ System32\ Drivers\ Etc\ hosts ) It will be in different places on different systems (Mac and Linux will have different directory names).
Normally the hosts file is fairly empty but by setting IP address of 127.0.0.1 (a special address for your own PC) it would mean lots of adverts would not be shown, and the PC would never send any request for a graphic image or web page off to the internet link (each request is part of your 20 GB limit).
In my hosts file right now I have:
127.0.0.1 pagead2.googlesyndication.com
127.0.0.1 www.google-analytics.com
127.0.0.1 www.partypoker.com
127.0.0.1 promo.clubxlive.com
127.0.0.1 ads.webcamclub.com
127.0.0.1 www2.leadingedgecash.com
127.0.0.1 ads.maroonspider.com
127.0.0.1 view.atdmt.com
127.0.0.1 ad-emea.doubleclick.net
127.0.0.1 smp.adviva.net
127.0.0.1 impgb.tradedoubler.com
127.0.0.1 www.adloader.com
127.0.0.1 track.adform.net
127.0.0.1 statse.webtrendslive.com
127.0.0.1 t.mookie1.com
127.0.0.1 s0.2mdn.net
127.0.0.1 ads.guardian.co.uk
127.0.0.1 adimage.guardian.co.uk
127.0.0.1 hits.gureoi rt.co.uk
and dozens more. There are websites which just track the thousands of advertising sites.
Of course, one problem with this approach is that if you see some advertising text link and click it, you might get a 'page not found' error because your PC will call up '127.0.0.1' and cannot find the web page.
Sorry if that seems too technical... I am sure there are one or two guides about using DNS to block adverts ad will find some for you if you wish.