After fixing HTTP/2 (H2) upload speed bugs in 2023 (Bug 1596576), Mozilla's Networking team started to focus on HTTP/3 (H3) upload speed (Bug 1852924).

The H3 upload speed problem is inherently different from the one in H2. H2 benefits from the mature, operating system-level TCP networking stack. H3 on the other hand is build on top of QUIC, which lacks an operating system API. All networking operations have to be handled in user space. This talk is about the journey of discovering and fixing various H3 upload speed bugs in Firefox using Wireshark, logging, i/o-graphs and more.