Openbsd 6.3 features Firefox 59.0.2 in lieu of 56 (or 55, can't remember). This new Firefox version ignores the
general.buildId.override value in about
:config. It leaks the actual buildId: the exact build date and time, down to precise seconds.
I tested this behavior at
https://browserleaks.com/javascript
Mozilla will not let me register for an account at this time, so I wanted to let the community know.
I intend to file a bug report with OpenBSD, although I don't think there's much the developers can do.
In the meantime, firefox-esr is available, but many useful applications, including uMatrix, are not available on pre-quantum versions. This is not only annoying, but insecure as well.