I have updated to Firefox 57, I don't know what is working behind the scenes, but it got a lot slower in my ubuntu machine. Before there were some incremental page loading while downloading other assets, but now, some of the sites are loading slower than before.some of them are:
http://store.steampowered.com/
https://gist.github.com/
maybe my configs are not correct or I should switch off something, but Firefox got worse