Firefox only uses OBS virtual camera
If you no longer need the virtual camera, you can uninstall the virtual camera plugin (whether it was installed as a separate plugin or as part of an OBS update) using the following commands in the terminal:
sudo rm -rf /Library/CoreMediaIO/Plug-Ins/DAL/obs-mac-virtualcam.plugin
sudo rm -rf /Library/Application\ Support/obs-studio/plugins/obs-mac-virtualcam

Once that is done, make sure to close OBS and restart (quit and open) Firefox.