Firefox allows you to use a proxy on Windows and macOS without additional extensions. On Android, you can use a proxy only via an extension (for example, FoxyProxy) or at the OS level. On iPhone, you can’t set up a proxy directly through Firefox, but you can configure a proxy via iOS system settings.
Setting Up a Proxy in Firefox on Windows and macOS
Go to Firefox settings and scroll down to the bottom of the page to “Network Settings.”