Created attachment 8842227 [details] speed_of_me_comparisons.zip I'm on a brand-new 100Mbs link (up/down). When testing on http://speedof.me and similar HTTP based network speed testing sites, I'm seeing Firefox upload speeds top out at about 45 Mbps. Chrome has little problem uploading at 75-100 Mbps. This is using a clean profile for nightly. I've attached a zip file containing a PNG and CSV of the data from speedof.me. These are both using the same machine and pointing to the same end servers. The Browser Console and Web Console show one potentially related error message: "Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user’s experience. For more help http://xhr.spec.whatwg.org"
And now it's working correctly. Going to chalk it up as a temporary glitch and close this bug.