Tested Mullvad Browser through ProxyChains4 (SOCKS4/5 → Tor → NordVPN) on BashLab OS.
The VPN IP stayed visible, seems Mullvad Browser may bypass ProxyChains’ LD_PRELOAD layer. Not fully confirmed, but consistent.
Firefox, instead, shows no IP leaks.
⚠️ Avoid using Mullvad Browser behind ProxyChains4 on BashLab OS.