This is an awesome Add-on! Works like magic! My main concern is if someone has the same first name as my deadname while writing an e-mail or such, so maybe a sort of toggle for now might be a good idea for people with common names? Restricting it to work differently on popular messaging apps or maybe even adding a toggle for people who are more out as apposed to those who are not and being able to specify websites where it doesn't work?
And about hiding the icon, for those concerned about that, you can pin it to the "Overflow Menu" and that will make it harder to see as you'll have to click something first. So an option until a stealth version comes out. <3