You can add it as a right-click option to Firefox:

• Install: webextension.org/…/external-application-button.ht…
• In the Extension options, add an application
• Name it something like “Open in YT-DLP: mp4”
• Set the exe name to the path of a local copy of yt-dlp
• Set the arguments to be -S res,ext:mp4:m4a --recode mp4 [HREF]
• Under the Placement options -> Context Menu -> select “All Contexts”

You should now be able to right click on a YouTube page and choose “External Application launcher -> Open in YT-DLP: mp4”

If you want audio only, add another application in the extension settings and use the arguments -f ba[ext=m4a] [HREF] for m4a files.