Backport #44394: Do not read default prefs from /etc/firefox

⬅️ Backport Patchset

Bookkeeping

Issue(s)

• #44394 (closed)
• mullvad-browser#xyz
• https://bugzilla.mozilla.org/show_bug.cgi?id=2016052

Merge Request(s)

• tor-browser!xxxx
• Upstream commit: 7a8cc12b
• Our commit to enable this: a2fa7105

Target Channels

• Alpha
• Stable
• Legacy

Notes

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