Most likely that is for the Sync service per the .us-west-2. part of that URL.

Are you / were you using Sync?

You probably won't get any connections at all, when Firefox is closed; but I wonder where you are seeing that Firefox is trying to connect, period?

You've asked about EC2 connections quite a few times.
We do not know all the ins and outs of the servers that Mozilla uses to store specific data files and aren't able to provide the answers you are looking (probably also outside the scope of this forum).

• /search/advanced?q=ec2&asked_by=&answered_by...
• /questions/1294511 Unable to identify connections to 104.16.249.249 & ec2-34-223-130-205.us-west-2.compute.amazonaws.com

All I know is that these are special servers that can be setup for specific purposes.

• https://aws.amazon.com/ec2/
• https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/concepts.html

the-edmeister, I am not using the sync service. How would I verify if the sync service was active ?

this is a security problem if a browser connects to a service the cannot be identified! Especially since no services from the browser are being used.