Firefox 57 still doesn't work!!
From Firefox 55 through to 57, the same trouble continues, so this question is still valid. This issue only applies to Raspberry Pi running Ubuntu.
For those who need a working version and got as frustrated as I did, here is a workaround. There does not seem to be any way to fix this issue, except by downgrading Firefox.
You can use Synaptic to force an old version of Firefox, by selecting the Firefox package, then go to
Package on the top bar, click
Force version and click
apply. Afterwards you must lock the package by going to
Package and selecting
Lock version. Note that you will only be offered Firefox 45.0.2.
You can also use apt from the terminal with something like:
sudo apt-get install firefox=45.0.2+build1-0ubuntu1 (You can install other versions like this, you just have to get the build name exacly right)
Lock the version with
sudo apt-mark hold firefox
Alternatively, you could download the version you want from here and install it manually. Firefox automatically installs new versions, so you will have to disable this.