works alright with firefox 57 32 and/or 64 provided one doesn't tinker with the optional settings or else the behaviour becomes totally random. So, if you want to have a fixed zoom (page and text) on all websites, do untick all unnecessary boxes (like autofit, CSS, image scaling...). That done, it is ok and you get (at long last) this most useful option back again on firefox 57.
and btw, beware of other zoom page extensions: one of them (rebranding of a very famous one) is ripe with scamware, installing the infamous yahoo search option that you can get rid of only by uninstalling entirely firefox, deleting its folders, cleaning the registry and reinstall it from scratch. Gross.

BUT: after a few hours, I discovered (sadly) that this extension stole the focus on google search to install the Russian Yandex search module. And as usual, same crap: you have to uninstall the extension then clean completely firefox, meaning uninstalling it and reinstalling it from scratch (same old story).