Outdated Instructions for Installing From Source in CONTRIBUTING.md

[moonmaster36]

Describe the bug

In the Installing From Source instructions, I noticed that some of the instructions are outdated and unclear.

For example, the original Firefox instructions are to navigate to
about:debugging in your browser. However, the actual URL to navigate to is
about:debugging#/runtime/this-firefox.

To Reproduce

Steps to reproduce the behavior:

1. Go to Installing From Source
2. Navigate to the On Firefox section of Installing From Source
3. Enter about:debugging in the URL bar
4. Try to click Load Temporary Add-on, but there is no such button at about:debugging
   
   Instead, you will find the Load Temporary Add-on button after clicking on This Firefox on the left hand side.
   

Browser and Vimium version

• Firefox
  • Firefox Version: 126.0
  • OS: Windows_NT 10.0 19045

Browser and Vimium version

• Firefox
  • Firefox Version: 126.0
  • OS: Windows_NT 10.0 19045

[philc]

Thank you. Fixed in 76fea7a.