Don't even bother downloading the latest version of X-notifier it's horrible. I just tested the 3.5.23 on the latest Firefox 53.0 (64.bit) and it's working great. Here are some tips you should do after you reinstalled 3.5.23.

Simply open Firefox click tools>addons>Extentions>
Click on the gear symbol> Then Uncheck "Update add-ons Automatically".

Otherwise If you don't Firefox will Automatically install the latest version again.

While your there you might as well go to Advanced>Update>"Check for Updates,But let you choose whether to install them". This will Prevent Firefox from automatically updated the latest version until your ready to install it.

Can't get it to work after the update.

I was shocked that after the update everything was removed, the settings disappeared, I could not configure, import, export did not work, the developer acted disgustingly, it's good that I have a habit of doing backups. I rolled it back to the previous version, turned off the updates, put the read-only attribute on the add-on. The latest normal version is 3.5.23 and still works in 2019.

Я был шокирован что после обновления всё удалилось, настройки исчезли, настроить не получается, импорт, экспорт не работает, разработчик поступил отвратительно, как хорошо что у меня есть привычка делать резервные копии. Откатил на предыдущую версию, отключил обновления, поставил на дополнении атрибут только для чтения. Последняя нормальная версия 3.5.23 и до сих пор работает в 2019 году.

I had a computer with the old version, I was amazed that it was working PERFECT. But it updated without my permission and now it does NOT work, just like in my other computers. WHY did you do this? Why ruined something that was working just fine? Why put auto update only to mess it up? Neve heard of "If it ain't broke, don't fix it!"?
Now it needs fixing. Please, fix it, it s REALLY simple. Just roll back to an older version and IT WILL WORK!! PLEASE WAKE UP AND HELP.

Every single feature of 4.0 is broken. New mail isn't notified on any accounts (Gmail, Yahoo), account names don't correspond to their correct accounts, etc. Please revert back to 3.5.23 and start from scratch.

The main purpose to install the plugin is the case that someone has many accounts. The script in gmail does not distinguish the regular emails from Social or Promotions anymore ("Notify inbox only" doesn't work at least the last 4-5 versions o Firefox). So it is not very useful.

Works for a while then fails !

Does not work anymore for Firefox and developers does not take any action to make it work again. Regrettable!

8/2019 update: it looks like support for this, once tremendously useful, extension has lapsed.

After the recent update to the latest Firefox version, the browser kept on logging me out of all Google accounts every few seconds - which very nearly drove me nuts. This problem was not replicated on Chrome, so, back on Firefox, I disabled X-Notifier and the issue disappeared immediately.

If you are experiencing a similar problem, I suggest that you disable X-Notifier and see whether it persists. I did try reporting a bug, but the linked Forum and Twitter account haven't been updated since 2017. So I've now completely removed X-Notifier and won't be using it again.
[2017]
The change in layout I can live with. But having to re-enter ALL data for all e-mail accounts every time either the add-on or Firefox installs a new update is a real pain. Already looking for an alternative. Shame, because it was a really useful add-on.

This used to be a must-have extension--you clicked on the icon and it opened up your email account. . .never a blip or problem. Now, it won't open your account automatically no matter how you set the options, and it randomly just loses your account information every month or so. If you absolutely can't live without this, resign yourself to constant irritations and inconveniences. I'm now looking for a replacement after using it almost since it first came out.

With the latest version at the time of this review (4.0.2), it works fine for checking email, but the UI is still broken.
There are 2 major issues.
1: You still cannot access the email with a single click, instead a single click now opens a menu where you must then select the account you want to open. this comes up even if you only have 1 account added to the extension, this you have an extra unnecessary step.
2: The icon for the latest version is completely ruined, it is literally 2.5 times the size of the older version, and number representing the unread mail, covers other content on the screen. Thus if added to a bottom bat (adddon bar) then the number stick out over the bar and covers content on any web page you are looking at. And if you have it at the top, it will cover content from other buttons you may have.

The developer either intentionally ruined the UI, or is for some reason does not care enough to fix these bugs. I know that some changes are needed to work with the new way firefox will handle extensions in the future, but those changes does not mean that we need to have these 2 major bugs. I first reported these issues about 4 months ago, and with new versions, the developer has actually made it worse

Has had spotty updates in the past and now is apparently no different. Worked good, now nothing. Its become a time waster.

Shitty version ... corrupted nice addition.
Suggest alternative.

X-notifier updated to version 4 without my permission, now Firefox (51) starts with an empty tab and all acounts are gone and cannot be saved. I used X-notifier for years and it worked flawless. But version 4 is a flop, please take it back.

Decided to check my addons today and seen that quite a few needed to be updated. I clicked ALL, and a few minutes after a refresh, I look up to see that my 15 email accounts have totally DISAPPEARED after the update! What an absolute horrible experience with an addon. Other that that, it would be a 5 STAR for me, I have used it for years.

There's a big problem with this plugin, it will lose any email accounts if you have browser set to clear history and data every time browser is closed.

Other than that, it does work just fine... I'm just tired of entering in my accounts every time.

Nothing displays in interface, seems to be totally broken, if it has anything to do with my ad blockers being enabled, then good riddance.

Przez wiele lat korzystałem , ale jak co kilka dni usuwa konta, to dziękuje za takie gówno, dziś znika z mojego komputera.

Años utilizando esta extensión y con fireshit 50 ya no funciona para nada!!!!!
1.Cuentas en rojo como si no la reconociera!!!!!
2.Clic en una de las cuenta y abre otra!!!!!!!!??????
3.Cierro fireshit 50 y cuando lo vuelvo a abrir, HA BORRADO TODAS MIS CUENTAS!!!!!!! NI UNA DISPONIBLE!!!!!!!!!!

Era el único sencillo y EFICIENTE!! Probe casi todas las otras extensiones y no son más que blabla, bugs and compañia, pajas mentales y INEFICIENCIA A GOGO.....

FUNCIONA A LA PERFECCIÓN EN CHROME!!!!!
O sea que después de muchos años,

ADIOS A FIREFOX!!!!!!!!!!!! Desintalo!!!!!!!

Used to love this program but now it's always in my face at bootup begging for money. Hate. Delete.

Cette extension a perdu toute sa fiabilité depuis le passage en version 4

Will you fix it or it's better to uninstall it?

Since the last update the pluggin is the same as in Chrome which is not efficient and useless. I used Firefox only to check my emails using this pluggin, now I will remove both Firefox and this useless pluggin

I used this extension to notify me of new email from an imap account. One day it just stopped working. If turns out the extension updated and removed that functionality with no notice. I had to re-install the old version and disable updated to get v3 back. This was the only extension I could find that supported imap. I'm very sad to know that this is no longer supported.

every time i install it all my data is erased, this version is a disaster, i am now using an older version until they update this mess!