disable per-installation profiles

Firefox ESR68 includes installation-specific profiles. See: https://bugzilla.mozilla.org/show_bug.cgi?id=1474285

With our current nightly builds, this causes an extra profile directory to be created and used (and probably our bundled browser profile is not be used on Linux and Windows).

We should figure out how to disable this feature or modify it to be compatible with the way we crate and use browser profiles).