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I’ve been troubleshooting a strange networking issue in Firefox that I haven’t been able to pin down, and it seems to be specific to Firefox’s network stack rather than my system or connection.

On several high-traffic sites that sit behind Cloudflare (Discord, Shopify storefronts, some SaaS dashboards), Firefox occasionally stalls during page load. The request shows up in the Network panel as pending for several seconds before finally completing. Sometimes the page partially renders and then hangs until the stalled requests finish.

A few things that make this confusing:

• The same sites load instantly in Chromium-based browsers on the same machine and network • The issue persists even in Firefox Troubleshoot Mode with extensions disabled • DNS over HTTPS on/off does not appear to change behavior • Packet captures show the TCP connection is established quickly, but some HTTP/2 streams appear to wait before data begins flowing • If I force HTTP/1.1 using a proxy for testing, the issue disappears

System details:

OS: Windows 11 Firefox: latest stable release Connection: fiber, low latency, no packet loss locally No system proxy configured

Things I’ve already tried:

• Clearing cache and site data • New Firefox profile • Disabling hardware acceleration • Testing with network.http.http2.enabled toggled • Comparing HAR logs with Chromium

One thing I noticed in the Firefox networking logs is that multiplexed streams sometimes wait behind earlier streams longer than expected, almost like there’s some internal prioritization or queue behavior happening. I’m wondering whether this could be related to HTTP/2 prioritization logic, connection coalescing, or how Firefox handles Cloudflare edge endpoints.

Has anyone seen similar behavior or knows whether there are any advanced networking prefs in about:config worth experimenting with (connection limits, HTTP/2 concurrency, etc.)?

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