It does not work in FF version 84.0.2 and hasn't worked for a while.

I also confirm AddSearch no longer work as of Firefox version 78.6.1esr (64-bit)

I also confirm AddSearch no longer work as of Firefox version 83. I love this add-on, I hope Burning Moth updates it soon.

Was a fantastic add-on, unfortunately doesn't work with latest firefox (83.0)

It's not working.

This works most of the time, but lately this error appears and prevents saving the new search:

Firefox could not download the search plugin from:
http://api.burningmoth.com/1/OpenSearchDescription.xml?method=get&action=...

THANKS！

Unfortunately, quite a few obviously great custom searches can't be saved, for example: Google Image Search just returns a pop-up error box full of gibberish from the BurningMoth website.

It doesn't work on Oxford Dictionaries websites:-(

COOL!

I have searched a successor to "Add to Search Bar" add-on for a long time., which does not work for recent Firefox.
I found this is "the successor"!

As commented by someone, for some sites, this add-on does not seem to work.
But, it is the fault of the recent specification of Firefox, I guess.

I deeply thank the developer.

Does not work. Download Error Code fills the entire comp. screen. Should be suspended until repaired.

Gives a long error when adding search (Google.com, Steam Store.. etc.) now:
=============================================================
Firefox could not download the search plugin from:
http://api.burningmoth.com/1/OpenSearchDescription.xml?method=get&action=https%3A%2F%2Fwww.google.com%2Fsearch&query=q¶ms[num]=50¶ms[newwindow]=1¶ms[source]=hp¶ms[ei]=-absW4rVJsn0vAS7zbXwAQ¶ms[btnK]=Google%20Search¶ms[btnI]=I%27m%20Feeling%20Lucky&name=www.google.com&description=Search%20the%20world%27s%20information%2C%20including%20webpages%2C%20images%2C%20videos%20and%20more.%20Google%20has%20many%20special%20features%20to%20help%20you%20find%20exactly%20what%20you%27re%20looking%20for.&icon=data%3Aimage%2Fx-icon%3Bbase64%2CAAABAAIAEBAAAAEAIABoBAAAJgAAACAgAAABACAAqBAAAI4EAAAoAAAAEAAAACAAAAABACAAAAAAAAAEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAP%2F%2F%2FzD9%2Ff2W%2Ff392P39%2Ffn9%2Ff35%2Ff391%2F39%2FZT
=============================================================

The error is way too long. Some places like Dictionary.com don't give error but don't work despite being added to search bar.

new add to search

В Firefox 64.0b6 x64 под Windows 10 на каких-то сайтах работает, в вариантах быстрого поиска появляется новая кнопка, работающая так, как задумано. Что-то - нет, выдающее сообщение вида "Firefox не смог загрузить поисковый плагин с http://api.burningmoth.com/1/OpenSearchDescription.xml?method=get&action=https%3A%2F%2Fpetrovich.ru (...)".

PERFECT IN EVERY WAY! Thank you! I was looking for something ever since the last addon I used to love stopped working when I updated Firefox 3 versions ago. Thank you so much for this. I have been looking for months! Thank you !

This addon worked brilliantly, until now that is unfortunately. With my current FF 63.0 (UX) adding to FF's search bar locks up FF and requires reboot. Also tested in default fresh install with same result.

EDIT01: I have to correct my previous review (luckily!): it turns out that not this very helpful addon is the cause, but the search bar I attempted to add (i.e. the new version of the ixquick -- starpage.com, https://new.startpage.com/).
However, when, as a control, I tried other search bars with this addon, it worked without issues.

EDIT02: Have to edit my remarks again: even adding a top-tier search bar (e.g. duckduckgo.com) gives a massive error in FireFox, and requires, as others have also reported, a full reboot.
The issue seem to be in bg.js of this addon, where the addon calls back to the site of the developer:

[...]
/**
* Context menu item callback onClick.
* @since 1.0
*/
browser.menus.onClicked.addListener(( info, tab )=>{
[...]
// format API url ...
// @todo When we can do this another way like Data URI scheme, scrap this custom api call ...
var url = 'http://api.burningmoth.com/1/OpenSearchDescription.xml?' + toQueryString(data);

...which results in:
Firefox could not download the search plugin from:
http://api.burningmoth.com/1/OpenSearchDescription.xml?method=post&action=https%3A%2F%2Fduckduckgo.com%2Fhtml&query=q&name=DuckDuckGo&description=The%20Internet%20privacy%
[...]

Ah well... Guess that @todo never happened...