I think there’s a difference between chromium based browsers and gecko based browsers.
In gecko browsers websites can’t see what extensions you have installed so an extension can be detected (or better inferred) if it inject code and modify the web page or expose itself in some way.
So there are actually nuances.
Mullvad browser’s developer apparently confirms
It’s worth noting that the method doesn’t work for Firefox extensions. Firefox extension IDs are unique for each browser instance, making the web-accessible resources URL impossible to detect by third parties.