Chromium, last I tried it, by default sets the necessary HTTP timeout(s) to infinity if its DevTools is open.
My searches of Firefox docs/wiki, StackExchange, HN Algolia for a fix have come up empty.
Navigating to "about:config" and searching for "timeout" finds 27 different settings prefixed with the substring "network". Some are obviously in units of milliseconds, others perhaps whole seconds.
Anyone know which one (or what combination) might incant the necessary black magic?