Check to force the user’s browser to download metadata (lists of available plugins, tools etc.) rather than Jenkins itself doing it. Actual file downloads (plugins, tools) will still happen from Jenkins itself, but this can be used to at least see new metadata when Jenkins cannot access the Internet (but your own browser can, perhaps using some special proxy that Jenkins is not configured to use).

Use of browser mode is discouraged. It has robustness problems and has been limited to users with Overall/Administer.