Same for me, Firefox on Linux. Getting 403's
Does this work?
https://www.openstreetmap.org/#map=15/50.9349/-2.6296&layers=P
You should end up with a view of a small town in South Somerset, UK.You mention "I also have the same referrer" - I'm not too sure what that means.
It sounds like you might want to go easy on the funky stuff. Cookies are not totally evil!
> It's a feature of uMatrix that's enabled by default
And I don't think uMatrix is all that funky. It works on all other sites, after all(rarely I have to allow-all, but that seemingly does not include the Referer: header)