I used Waterfox long time ago, as a fast 64bit alternative to Firefox (although never noticed a significant speed difference). Now I think the only reason I'd still use it is because add-ons such as Vimperator, but I already switched to the less powerful Vim-fx.

I love the idea that projects such as this one or Pale Moon exist, but I find it hard for me to use them as a regular user, apart from sentimental reasons.