Menu app icon > Options/Preferences > General
Scroll down to the very bottom
click on 'Config Editor' button to open 'about:config'
Accept the risk.
In search type: pixels
Look for this line: layout.css.devPixelsPerPx
Double left click on that line to open the 'Enter string Value' box so you can edit the Value. The default value will be a negative number: -1.0
Enter a new positive number: 1.3 seems to work for me, then click on OK.
The effect is immediate, so you can modify that number until it looks ok for you. Image below as guide.