Add documentation on how to test on older firefox versions

What does this MR do?

Adds a guide on how to test things with an older version of firefox

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Conforms to the code review guidelines
  • Has been reviewed by a Frontend maintainer
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?