Firefox Sync stuck at "Working..."

Coming from Fedora to Debian Sid
Now Firefox Sync doesnt work

Update: I also cannot install addons via addons.mozilla.org
After tinkering around i noticed the browser's useragent is set to display as Netscape on Linux, which is strange

I have turned general.useragent.compatMode.firefox to true via about:config and it seems to work, i can use Firefox Sync and use addons now