Apologies if this is the wrong place to ask this because I'm at my wits end on this one. I work on an app that is used by a large amount of users. Starting october 14th around 7am (UTC+1) we started observing weird behavior where a small ammount of traffic (less that 1%) would have abnormal header values: 'custom-header-1':'[custom value], [custom value of a different header]'; 'custom-header-2':'[custome value of header 2], u=4'; 'sec-fetch-site':'cors'; 'priority':'same-origin'; 'sec-fetch-mode':'empty'; As you can see the values of these headers are mixed with one another in almost a deliberate manner (since it appended the u=4 with a comma separator and a space from the priority into the custom header 2 value). There is nothing in the client code that would do this kind of manipulation, and from examining the logs this only happens on Mozilla browser traffic starting version 144 (which was released on October 14th coincidentally). Not that this does not impact all Firefox traffic. In fact, just a tiny fraction of traffic from random users and it's not even all of their traffic of an active session. Just random requests that then fail, are retried and the abnormal headers are gone. I doubt the fault lies in firefox. If I had to hazard a guess it's probably some extension acting up, but the timing coinciding with the release of 144 give me the only troubleshooting path I have right now since contacting the users is not possible. We've seen it happen 145 as well. I'm not really looking for a solution here, just a shot in the dark if someone else has experienced this or has some insight as of what this could be.