Opera back in and before the Presto days just had a switcher in the menu. (As well as a “cached images only” toggle, which was why I used it in the first place, but I digress.)
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*