Firefox does block pop ups but there are links that like to open new Windows to the chagrin of some users. To disable webpages or links from opening up new windows modify these 3 settings from about :config -

browser.block.target_new_window = true
browser.tabs.opentabfor.middleclick = true
browser.tabs.opentabfor.urlbar = true

This will not be perfect though it should result in fewer open Windows due to links.