There's many forms of timeout, are you after the connection timeout, request timeout or time to live (time before TCP connection stops).
The default TimeToLive on Firefox is 115s (network.http.keep-alive.timeout)
The default connection timeout on Firefox is 250s (network.http.connection-retry-timeout)
The default request timeout for Firefox is 30s (network.http.pipelining.read-timeout).
The time it takes to do an HttpRequest depends on if a connection has been made this has to be within 250s which I'm guessing you're not after. You're probably after the request timeout which I think is 30,000ms (30s) so to conclude I'd say it's timing out with a connection time out that's why you got a response back after ~150s though I haven't really tested this.
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