No one mentioned what environment they’re running on.
My main environment are Linux machines (desktop/laptops running Tumbleweed).
(BTW - this also works on Chrome)
To change the UA for, let’s say, desktop Linux or Windows, do this
a) Menu button => More tools => Developer tools
… In the upper right of Dev Tools widow is a
b) “three-dot menu” => More Tools => Network Conditions
Network Conditions tab is displayed at bottom third of window
(it’s scroll-able or you can move that window pane up).
c) For User agent, un-check Select Automatically, then fill in whatever UA string you’d like in the Custom field.
(a)

(b)

( c )
