Iirc there are add-ons for it, I'd calllp that easy. Also there's the about:config way but I guess this may be considered difficult.
This is only for responsive design mode in which FF mimics iPhone (Mozilla/5.0 (iPhone; CPU iPhone OS 12_0 like Mac OS X) AppleWebKit/604.1.38 (KHTML, like Gecko) Version/12.0 Mobile/15A372 Safari/604.1)
There is no straight way to just change the useragent like I could do with IE10 *grin*