This post will guide you through unlocking the sidebar width in Firefox. It will let you resize it beyond what is possible by default. We will be setting both the maximum, and the minimum width. You will have a chance to customize these values once at the right step.

Why Would Anyone Want to Do This?

  • To enhance the use of the Firefox Side View extension.
  • To compliment the use of one of my Firefox sidebar extensions.

Step-by-Step Instructions

  1. In a new tab, navigate to about:support.
  2. Under Application Basics, find Profile Folder.
  3. Locate and click the Open Folder button next to it. It will be next to an address similar to:1
  1. Inside your Firefox Profile Folder, create a new folder named chrome.
  2. Inside the newly created chrome folder, create a new file named userChrome.css.
  3. Copy the following code,2 paste as content and save:
#sidebar-box {
  max-width: 40% !important;
  min-width: 300px !important;
  1. You may change the units to any length you find more convenient.
  2. Finally, in a new tab, navigate to about:config, and search for:
  1. Change its value to true.
  2. Restart Firefox and test it out!


  1. In windows, %appdata% is equivalent to C:\Users\{username}\AppData\Roaming.

  2. After Firefox 107, #sidebar was deprecated, and #sidebar-box was introduced as a replacement.