Great and simple Rated 5 out of 5 stars

Does exactly what I wanted, thanks for the great work

Great but needs improvements Rated 4 out of 5 stars

As already mentioned dropdown of values history will be great. Also it would be great to set specific values for specific domains.

You definitely need drop-down menu instead of that Rated 3 out of 5 stars

And ability to add presets to the options for needed combinations. Use that presets in the drop-down menu.

Awesome, quick question Rated 5 out of 5 stars

Works great but question is why are there two values? For example en-US, en?

This review is for a previous version of the add-on (0.9.1). 

The Accept-Language header accepts a list of possible languages... "en-US, en" is probably superfluous, but something like "en-US, en;q=0.9, fr;q=0.8" would mean "I want American English most, then any other English, but would accept French if English isn't available."

There's more info on MDN at https://developer.mozilla.org/docs/Web/HTTP/Content_negotiation#The_Accept-Language_header

Great when it works Rated 4 out of 5 stars

It occasionally disappears from my toolbar, and the customize menu can't bring it back. Not sure what triggers it to come back again.

This review is for a previous version of the add-on (0.9.1). 

I've updated it to a WebExtension, which should fix that.

simple and useful! Rated 5 out of 5 stars

...but, as a lazy user, living in 2015, it would be nice to have a selection pulldown to choose from.
Maybe some user editable pre selections, I can choose from...

This review is for a previous version of the add-on (0.9.1). 

Rated 4 out of 5 stars

Works great, but we need a way to get back to the original value

This review is for a previous version of the add-on (initial.rev102.1-signed). 

Done! In 0.9.1, if you erase the value in the entry box and hit return, it will reset your Accept-Languages header to your locale's default.

Does exactly what you'd want! Rated 5 out of 5 stars

Does what it says, but a few small tweaks would greatly improve the user experience.

The ability to reset the value back to what it was before I typed something in would be nice. Also an option to refresh the page after changing the value would be hugely helpful!

This review is for a previous version of the add-on (initial.rev102.1-signed). 

Thanks for the ideas -- I didn't realize people other than myself were relying on this :) I'll definitely put some time into the UI/UX soon.