And sadly all android webviews are Chrome (and with a lot of additional effort at least Chromium based). Mozilla should release a webview and fight for signature inclusion.
The parent poster is talking about system webviews. They're used when any app wants to show web content embedded inside their apps, and yeah there's no Firefox option for those.
If you don't care about any of the extra functionality, you can configure it to always open your preferred browser and convert Custom Tabs intents to regular ones. No root access required.