I love this add-on!
With the recent onset of Apple enforcing its "proprietary" right to use the emojis we are all used to, Slack, et al were forced to switch to their own, non-Apple Color Emoji typeface -- and if you're like me, they're HIDEOUS emojis. This extension allows me to permanently spoof the user agent to an iOS device for specific sites where I use emojis a lot. Like Slack, fb, etc.
When I was using Chrome browser, I could only achieve this by CTRL+I and de-toggling "Select automatically" under the Network Conditions sub-menu inside dev tools. So this permanent fix in Firefox is exactly what I wanted.