On the pull request page, look at the checks for this pull request:

Click on one of the CI checks:

Then, scroll down and download one of the artifacts:

Theses links won't work unless you are signed into GitHub. You can bypass this restriction using https://nightly.link

See dedicated page

Go to about:debugging

Click "This Firefox"

Click "Load Temporary Add-on"

Extract the downloaded zip, and choose the manifest.json file inside it

Note: The extension will be automatically deleted when you close Firefox