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