This is placebo and untrue.

HTTP response timeout is unused when TCP-KeepAlive is enabled (which it is by default): https://searchfox.org/firefox-main/source/netwerk/protocol/h... (also see https://bugzilla.mozilla.org/show_bug.cgi?id=1024015) and Firefox only uses the value until the FIRST byte is received anyway: https://searchfox.org/firefox-main/source/netwerk/protocol/h...

>I do also suspect firefox does some hidden retries if it can before it considers the download to have failed

There is no such mechanism.