FireFox 58 does send the beacon when the browser is closed

rhertogh@ - Thanks for filing the issue...!!

Could you please provide a sample test file/url to test the issue from TE-end.
This will help us in triaging the issue further.

Thanks...!!

I'm no longer sure if this a bug.

When working on a sample I was thinking about the security implication that sending a beacon when the browser has been closed would have so I digged a little deeper in the sendBeacon definition and found some contradictory definitions. 

From https://developer.mozilla.org/en-US/docs/Web/API/Navigator/sendBeacon
"By using the sendBeacon() method, the data is transmitted asynchronously to the web server when the User Agent has an opportunity to do so"

https://www.w3.org/TR/beacon/
"The sendBeacon method does not provide special handling for offline storage or delivery. A beacon request is like any other request and may be combined with [SERVICE-WORKERS] to provide offline functionality where necessary."

So it looks like the statement on the mozilla website "transmitted asynchronously ... WHEN the User Agent has an opportunity to do so"
should be "transmitted asynchronously ... IF the User Agent has an opportunity to do so"

Following the w3 specification the behavior described in this issue would not be a bug.

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Thank you for reporting. I think that our current policy is that closing the browser cancels keepalive requests, and the feature is working as intended.

Note: Service worker is also terminated when the browser is closed.