You could do it with Cloud Functions, which "run backend code in response to events triggered by Firebase features and HTTPS requests", see the general doc here.
You could regularly trigger an HTTPS Cloud Functions that would:
- Call the REST API, e.g. with Request (https://github.com/request/request)
- Based on the response of the REST API, send a notification with FCM.
To send a FCM notification via Cloud Function, have a look a this official Firebase sample https://github.com/firebase/functions-samples/tree/master/fcm-notifications
To call your HTTPS Cloud Function regurlarly, you have to trigger it via http through a cron-job.
Have a look at: https://firebase.google.com/docs/functions/http-events and
https://www.youtube.com/watch?v=CbE2PzvAMxA. You could also use another online CRON service to call the HTTPS Function, like https://www.easycron.com/
Note that you would need to be on Blaze Plan to be able to query an outside service.