I found this page on the X-Notifier forum.
http://xnotifier.tobwithu.com/dp/node/3596
It says how to fix the issues. It was posted around 7.16.2017. It works for the most part. Need to follow carefully.

After many years of using this addon it seems as though all support for any version of FF below 57 has been dropped. That's sad because with the release of FF 57 mozilla will be losing a large portion of their user base.

Always loved this app.
ON occasion revisions and updates stop it from working but Mr Kang ALWAYS FIXED THEM.
Now, suddenly, it won't work, but I also notice that my gmail accounts will not stay signed in. Connection? solutions? Someone help !
Thanks.
P

muy buena la última actualizació. Me encantaría para otra versión futura que se puedan ver los asuntos de los correos y una o dos líneas de mensaje, al estilo de lo que permitía hasta hace 4 días el gmail checker. Gracias !!!

Version 3.5.23 pls fix yandex.ru script

now since version 4.2.16 is working fine again with gmail, Outlook, Aol, & Yahoo ... pretty nice Add-ons ... keep it rolling & thanks for all developments

Everyone seems to to directing their frustration at the developer. He has worked without much reward for many years to give you this.

It is Google and Firefox that keep breaking this great add-on. Send your anger to these 2 and thank Mr Kang for still trying to get fixes done though it sometimes takes a few versions to get things right again. Then in a few months Google, Yahoo, etc will break it again.
Personally I would give Mr Kang a 5 stars at all times.

The new version doen't work either. I have used the add-on for years and I really like it. it's a shame it broke.

Version 3.5.23
work on firefox 51 with multiprocess enabled

I've been using this add-on for years. I don't know why it suddenly stopped working. There is no normal menu, it doesn't check e-mail box, nothing but an icon. I'm sure it is not Firefox/Cyberfox to blame because there was no update meantime. What the heck is going on?!

I've pinned mine to 3.5.23. Best gmail notifier ever. It is a shame Mozilla is changing their standards yet again causing all this negative feedback. It is not his fault Mozilla has changed. When 3.5.23 no longer works on FF release, I will try it on FF ESR. Thank you for the best notifier prior to Mozilla changes.

I've just upgraded from Firefox v43.0.1 to v50.1.0. The loss of this irreplaceable extension—the best of its kind—hurts my heart. That said, the endless changes to Firefox have broken so many great extensions, I don't blame any developer for giving up on Firefox. I would too.

The good news is that there's a version of X-notifier for Chrome that works beautifully. I dislike Chrome, but have installed it for the sake of X-notifier.

To the developer, many thanks!

At first, I don't liked the new (Web) interface.
But then, I saw that there are no problems (apart from the need to re-import all of my accounts).
So finally, I'm OK with it.
I always use the latest Windows 10 (64-bit) & Firefox versions...

Je te remercie sincèrement, ça marche nickel !

If like me you use x-notifier to keep all your email sign-ins in one place you were mortified to find that updating to ver 4 resulted in all your sign ins gone. All is not lost. Reinstall version 3.5.23 from

http://xnotifier.tobwithu.com/history.php?br=ff

and then your logins all reappear. Export them from options, and then you can import them to the latest version. I wish this had been better handled, but x-notifier is so critical I am giving another 5 star review.

Now is the time to send some love in the form of a contribution to the author. He deals with major crap so we don't have to.... This was our turn...

Firefox has done this to so many good addons. If I wanted security I'd get off the internet. Chrome addons are inferior in action, google has succeeded in nothing except sandboxing your personal info for themselves to feast on. FF is a memory hog and memory sieve, but it is a hell of a lot more aesthetic than chrome

Go here: https://addons.mozilla.org/en-US/firefox/addon/xnotifier/versions/
download version 3.5.23.
Then go to your add-ons, find it, and in its options there is the possibility to turn off automatic updates.So it will work just fine :)

If anyone has destroyed anything, that's Mozilla with their decision to drop XUL, not tobwithu. I am frustrated, too, but please, read his notes before deciding to give it a bad rating. Also be aware that there is an issue with black text on dark themes that has been posted in forums.
Re-installing 3.5.x automatically brings your old account settings back (logins, scripts, RSS, etc). - they're not lost, actually (make sure you disable automatic updates on the extension's "More" page).
While looking fwd to see scripting back (or a replacement) and Neo growing mature, I'll use 3.5 for as long as FFox allows it. (If you think it's time to say goodbye, it's maybe time to say goodbye to FFox... although I probably won't. Sad.)

I'm so sad , i have used xnotifier for about 6 years . the latest update 4.0 is awful and disappointing , can't get it work with my 8 accounts and I keep refreshing many times until get all my accounts logging in . Now it takes 4 ever and age to login to all accounts . I'm searching for alternatives. Good bey my sweaty . It's now a web-based and not an actual add-one i think so.
Developer's note
########################################################################
X-notifier 4.0 does not support user script because of the internal changes in Firefox.
########################################################################
It's ffox decision 2 destroy the xnotifier

Hola la version 4 es malisima he tenido que volver la version 3.5
Primero en heramientas complementos desactivar actualizaciones automaticamente y luego volves a instalar x notifier 3.5 que va fenomenal un saludo a todos

Works with my isp email and zoho email without adding scripts.

Used to be a very good extension.

Now ... Forget about it ... Maybe it is a good thing to get prepared for what will happen in next year (no more user scripts, no side bar/ window, etc.).
I really do not understand Mozilla's policy to align with Google Chrome's one, follow the "big brother"? Since when Google drives this game? Since 15 years ago, when you were not able to get a free Gmail account without an invitation? Is Firefox an open-source software that listen to the crowd, or to the other competitors? Firefox users want to have user scripts! I'm still using FF 3, yes Firefox version 3 for my bank transactions (and I never had an issue); what is this FF 50 version, a game? Let's go and get Chrome 56+ version? For those who want Chrome, let them to have it (the true Google Chrome!) For us, the others, we want Firefox WITH user scripts! ... Never mind ... Let's go back on track!

Old X-notifier version 3.5.23 does not work anymore with M$ servers (Outlook.com, Hotmail ...). I'd expect in a few weeks none of free services (Yahoo, Gmail, etc.) will work ...

New version 4.x.x? Without side window? No way! And it will delete the existing accounts ... Do a backup/ export ASAP! (Better do it for the entire Firefox! please see FEBE add-on, if you're not on portable version, aka www.portableapps.com).

Look somewhere else ...
Read Yahoo emails in Gmail? Is that secure?
Howard Email Notifier, POP Peeper , POPman ... as alternatives for Windows users?
Or Geary for Linux users ... (or Pantheon Mail) ... ?

Or setup your own mail server, maybe on a small Raspberry Pi?

Let me know what it will be your choice/ best next experience ...

Regards

The new version is really bad, sadly. I've been using this addon for years but now I have to look for an alternative.

-----------edit
Followed this instructions http://xnotifier.tobwithu.com/dp/node/3353 and installed the older version. Works great for now and i wont update mozilla only to keep this on. Thanks.