Please confirm that this bug exists in the most recent version (148.0.2).

It got even worse after updating to the latest version 148.0.2.

Try to enter about:config in the address bar and set network.http.http3.enable to false

Optionally set network.http.http2.send-buffer-size to 33554432

Then restart the browser.

Thank you for answer. Unfortunately, nothing has changed. The speed is the same.

We’ve been actively working on improving HTTP/3 upload performance over the past year. This includes optimizations such as fast UDP handling and reducing spurious retransmissions.

To better understand the performance gap, we built a benchmarking framework with a simulated network environment to measure upload for both Firefox and Chrome under controlled conditions.

In a 300 Mbps bandwidth / 40 ms RTT scenario:

• Chrome upload: ~190 Mbps
• Firefox upload: ~150 Mbps

These results show that there is still room for improvement, and we will continue working on closing the gap. However, they also indicate that Firefox is not two times slower than Chrome, as originally reported in this bug.

With Neqo release v0.22.0 that will land in version 149.0 we improved HTTP/3 upload performance for connections that see spurious losses, e.g. because of packet-reordering. We see about 11% of connections experiencing some degree of this in desktop telemetry and could see up to 2x upload throughput in local measurements and benchmarks, depending on the severity of packet-reordering. This is a win for upload stability and robustness.

Sorry, but speed two times slower than Chrome. I tryed many times.

Sorry, we don't have gap of this size in download, only in upload.