(tech details omitted here as believed to be a fairly common issue [perhaps: https://bugzilla.mozilla.org/show_bug.cgi?id=120679] although happy to provide some if asked) Sorry to raise this issue from about 6 or 7 years ago but it is quite a niggle - if not a potential privacy/security risk.. For info, I think this was fixed briefly about that time but returned in a slightly newer version. If you plan to fix this I should be most happy to await Debian developers to consider a newer version of Firefox ESR a "stable" package and release it to the likes of me. Many thanks and best wishes Dave C