Black Sides by Firefox user 14568023

This is meant to be used on websites, where there is too much white space on both sides of the window. I find it more comfortable when these areas are "switched off".

This extension adds left and right borders of custom width on the current website by injecting <div> overlay.

How to use it
Open addon popup by clicking on its icon. Click "+" or scroll up with mouse within popup area or:

hotkeys:
F7: thinner borders
F8: wider borders
F9: on/off

Settings for each website are saved in local storage. Its my first addon.