How to make side panel resizable:
Updated instructions for Feb 2024 (I'm on Firefox version 123)

- Go to about:support
- in Application Basics section find row called Profile Directory, click on Open Directory button
- Close all firefox running instances
- In that folder, find xulstore.json and open it with a text editor
- Find where it says "sidebar-box":{
- inside those brackets look for "style":
- inside this find width: and replace with the following:
width: 50%; max-width: none !important; min-width: 0px !important;

- if you did it correctly it should look like this: "style":"-moz-box-ordinal-group: 3; order: 1; width: 50%; max-width: none !important; min-width: 0px !important;","checked":"true"

- save the file and close, open Firefox and have fun :)