You are right. The bug in BW Firefox add-on (at least for macOS) is that it assumes that this folder exist - which often is the case as it could be created by other add-ons. It attempts to write there some json config file when browser integration is enabled - hence after creating the folder it is needed to re-enable this setting.
So for unlucky ones full workaround at the moment is:
- Create missing folder
~/Library/Application Support/Mozilla/NativeMessagingHosts
- In desktop app disable & re-enable the browser integration setting
- for macOS Mojave and Catalina create also
~/Library/Containers/com.bitwarden.desktop/Data/tmp
if does not exist