If you have one or more other web apps (Angular/React/HTML) that are deployed to a hosting site on your Firebase project, simply add them to your workspace as usual using the standard nx g app generators.
Then just update your firebase.json or firebase.appname.json hosting configuration to point to the dist/apps/<webapp> where your web app build output is.
You can then run the Firebase CLI as usual to deploy the site:
firebase deploy --only hosting --config firebase.appname.json
Or
nx deploy appname --only hosting
If you deploy static websites to Firebase Hosting (that do not need to get built by Nx), just create a folder in your apps directory (rather than generate an Nx web app) and put your content in that folder. Then update the hosting section of your firebase.json or firebase.appname.json to simply point directly to this folder.
The firebase CLI hosting deploy command above will just upload the static content as required.
An application generated by Nx-Firebase is by default configured to host content in the apps/appname/public directory.