Adding mail.ru and inbox.ru to x-notifier

Mozilla (Firefox) is forcing a big rewrite onto all the add-on developers by changing its API or Application Programming Interface for Firefox 50+.  If you had the Firefox global "automatic updates" for addons turned on... and upgraded to FF 50+ ... you probably got X-Notifier auto-updated, but accounts could not be carried over (see below). (Each add-on's "More" page in the Add-ons list has its own update settings, which default to "default" ...the Firefox global setting.)

- So... eventually we users who upgrade Firefox to version 50+ will have to transition to XN version 4.x. Unfortunately, the API changes mean dropping support for POP3/IMAP and custom scripts, among other things. However... you can, if you want, for the time being, continue to use version 3.5.23 on FF 50+ untill Mozilla locks down the new changes for addons.

Others have downgraded to 3.5.23, and... recovered their account definitions... 
You can try reinstalling X-Notifier 3.5.23 ...it can be downloaded here:  https://addons.mozilla.org/en-US/firefox/addon/xnotifier/versions
...and, after reinstalling, you should then turn off automatic updates (just for X-Notifier) by going to FF Tools, Addons, find X-Notifier, click 'More', scroll down to 'Automatic Updates' and select 'Off'... so that it doesn't auto-update to 4.02 right away.  Later you may want to set that setting back to "Default" for automatic updates... which won't have this API changing problem (for a long while anyway).
- THEN, in 3.5.23, be sure to Export (backup) your account definitions by going to X-Notifier 'Options', scroll down and click 'Export'.
After doing that... if you want, you can update back to 4.02... (as it's necessary to Import your accounts, or else redefine them from scratch, as they are not autmatically transferred during the update from 3.5.23 to 4.05 ...because data structures have changed)... but, again, there are -no- custom scripts in 4.x.
- NOTE:  After 'Add'ing accounts in XN 4.02 ...be sure to (scroll down, if necessary, and) click "Save"  ...or they won't be.

XN 4.02 supports hotmail/outlook/live.com accounts, gmails, yahoo, and AOL... and RSS (my imported RSS definitions work for me anyway).  

Or... you can switch to X-Notifier Neo for Firefox... or... switch to Chrome and X-Notifier for Chrome (custom scripts supported, but no POP3/IMAP) ...or... Safari, on a Mac (no longer available for Windows).

Just FYI, you can download the older 49.0.2 version of Firefox; 
https://support.mozilla.org/en-US/kb/install-older-version-of-firefox

The developer of X-Notifier 4.x has said he cannot program for custom scripts due to limitations imposed by the new Mozilla/Firefox API (application programming interface).  If Mozilla/Firefox doesn't make it possible, X-Notifier can't do it.

I expect he will only 'embed' (include within the program itself) access to webmail that he can program for, that are most popular globally... but I have no idea what he can include by default.
Maybe Mozilla/Firefox will change in the future.  

Chrome and Safari, maybe Opera, can still do custom scripts.

I use all the browsers, somewhat (except Safari is not updated anymore for Windows)... and I use those email 'services', among several others.

I would think SeaMonkey/Thunderbird will need continued support of 3.x version, if there's to be any new X-Notifier for them... otherwise, they may be frozen on 3.5.23.

SeaMonkey includes the Internet browser/Composer as its main function, I think... but it does do email.
Thunderbird is email with the Lightning calendar!