browser plugin (firefox and ie)

I'm not sure this is possible but maybe a creative programmer can figure this out. Its a browser plugin and once installed whenever i go to any site it blocks ads from that site. (i know Ad Blocker does that). But i have a little twist to it... To place my ads into the place where the other ad was. So if i come to [login to view URL] and there are ads there from [login to view URL] this software will replace it and show ads from mydomain.com. The tricky part is how to figure out the right size. Because an ad that is 450x350 should be replaced by an ad of the same size. And if you try to replace it with 468x60 ad it will look very bad. This has to work for all sites and for all banner sizes. Let me know your idea and we can discuss it in more details.