when firefox 38 sent FIN,ACK after server ack an http get from firefox?

问题

see attached wireshark log:(pls click it to make it bigger enough for view):

in packet 37 , client sent a http get to server
in packet 38 , server response ack
in packet 39, after client wait 5 seconds, client sent a [FIN,ACK]

client is a Firefox 38 on a win 7 system.

my question is: why client sent a FIN,ACK after 5 seconds?

It is because after wait 5 seconds, nothing come from server? if my guess true, is there any setting in Firefox to extend this, as server is slow and 5 seconds is short.

if not true, is it just because client (firefox browser )has some issue which cause client to sent FIN ,ACK to close connection? OR could be some server issue? OR firefox issue?

we encounter this issue intermittently when server performance is slow. Server people say it is not a server issue, and client people say it is not application client issue?

Could it be possible just firefox issue, or some extension on firefox cause client sent FIN,ACK?

the most important question:

from the wireshark, it is reasonable that server people say no problem at server because client issued a fin,ack?
Is there any firefox setting to change this behavior?

thanks in advance for anybody who knows TCP/IP.