uMatrix window too small in Firefox 56

Problem

I upgraded to Firefox 56 and immediately noticed that the uMatrix dashboard window was too small. I could not view some of the columns to the right.

Solution

The solution seems to be related to the Photon UI of Firefox. Go to about:config from the location bar and set the config option browser.photon.structure.enabled to true. The effect is immediate and your uMatrix dashboard should be back to full size again.

Tried with: Ubuntu 16.04