Firefox for Android does not provide a UI option for users to to configure Firefox to use PAC file for proxy settings, but we can use the about:config page to achieve this goal.
Type about:config into your Firefox address bar, and you'll see a list of options below. In the search box, type proxy to list all options related to proxy. There are two notable options listed here: network.proxy.type and network.proxy.autoconfig_url. Change network.proxy.type to 2 and network.proxy.autoconfig_url to the URL of the PAC file (Tap on the edit box below the configuration option twice to start edit the option). Close the about:config page and now Firefox should use the PAC file for proxy.
To configure the proxy of Firefox on Android using a given proxy server and port, please see this article.