Once you install the app it will give you a few options of keywords that the extension will block. Be careful, though! I removed "cute" and "too cute" because — and I don't know about your facebook feed — my feed is bombarded with cute animals and I would like to keep them there.
Next you can choose what kind of photos you want the babies to be replaced with. The default is Instagram's #cats tag but you can choose any tag OR any RSS feed you prefer.
Voilà! That's it. Mashable has a screencap of the extension in action):
Fair warning: you still may find a stray baby here and there — but for those who are so over the constant babyspam, this comes as a huge relief.