Mozilla recently tested the most popular extension Adblock Plus. It has approximately 19 million users.
“The difference is significant, especially under the right circumstances. Adblock Plus adds an overhead to Firefox's memory usage of about 60-70 Megabyte. While that may be a lot depending on the installed memory on the computer, the difference can widen quickly under certain circumstances. For every iframe that gets loaded in the browser, four additional Megabytes are added to the browser's memory usage. That does not appear to be much, but if you consider that some sites may make use of many iframes, it can grow quickly.” – gHacks
On a regular machine, this isn’t a bad trade off. On a low end machine, the picture is quite the opposite, and may really slow things down to a crawl.
So, Adblock Plus will hopefully get some love from its devs. In the meantime, if you hear complaints about Firefox being a memory hog, in truth it isn’t.
It’s Adblock Plus and the site you’re on.