[Firefox compat] Pages cannot be loaded with enabled Windscribe extension

[TheHasagi]

Pages won't load with enabled Windscribe extension.

Steps to reproduce

1. Enable AdGuard
2. Open Firefox and enable Windscribe extension

Expected behaviour

Everything should be in order

Actual behaviour

Pages can't be loaded

Your environment

• Environment name and version: (e.g. Firefox 67.0)
• Operating system and version: (e.g. Windows 10, v.1703, build 15063.413)

[adbuker]

@TheHasagi , please try it with the latest nightly build

[sfionov]

Issue moved to AdguardTeam/CoreLibs #960 via ZenHub