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.
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