I noted that when installing a Safari Extension from TestFlight, it is installed directly into Safari with no choice of browser to target.
I honestly don’t remember, but expect that’s so from the App Store.
Perhaps similar to Apple having loosened its grip on default applications, they need to not presume and enforce that the only browser to install extensions to is Safari.
There are some excellent extensions, which are among the only occasions and reasons I use Safari, including
Unobstruct https://apps.apple.com/app/id1255281426
Vinegar https://apps.apple.com/app/id1591303229
Baking Soda https://apps.apple.com/app/id1601151613
StopTheMadness Pro https://apps.apple.com/app/id6471380298
Makeover https://apps.apple.com/app/id1602361167
So yeah, I’m into extensions!