If you want to use a PAC file, set Proxy mode to "Global proxy setting" and specify the URL of the PAC file under Network > Proxy > Automatic proxy configuration.
Proxy mode
Specifies if a proxy should be used.
Possible options:
Off*: A proxy is disabled. The direct connection to the Internet is used.
Global proxy setting: The proxy configured under Network > Proxy is used, see Proxy.
Session specific proxy: The proxy configuration specified under Proxy host and Proxy port is used.
The following fields are active if Proxy mode is set to "Session specific proxy":