User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:65.0) Gecko/20100101 Firefox/65.0 Steps to reproduce: 1. Install fresh Firefox 65 onwards (tested 65b3, 65b7, 66a1). 2. Setup some forwarding proxy server (tested Apache/2.4.37, tinyproxy 1.10.0) within Firefox with using for all protocols. 3. Go to address https://doozer.io/tvheadend/tvheadend Tested on Firefox 64, Chrome, Edge without any trouble. Actual results: Websocket cannot be estabilished. No branches shown on the doozer website. Expected results: Websocket should be estabilished and some branches loaded and shown.

Could be related to bug 1514688 Reporter: Could you run our mozregression tool to find the change that caused this ? - https://mozilla.github.io/mozregression/ I would do it but I'm currently not at home and do not have a proxy server available.

Regression tests (by the way very nice tool ;)) found this commit... Bug 1434137 - Implement websockets over http/2 - RFC 8441 r=michal,dragana https://tools.ietf.org/html/rfc8441 This uses our existing http/2 CONNECT infrastructure (modified) to enable the new extended CONNECT form defined by 8441, and pretend for the websocket's sake that an http/2 stream is actually a socket. From the websocket's point of view, this is relatively non-invasive - a few things have changed (http response code, absence of some headers) versus http/1.1 websockets, but for the most part, the websocket code doesn't care. Differential Revision: https://phabricator.services.mozilla.com/D8016

Thank you very much for doing the work ! This could be indeed a dupe of bug 1514688 which will be hopefully fixed in the Firefox65 final version.