Webmail Notifier has been one of my essential extensions for years.
After the xul apocalypse, I switched to Palemoon as my main (and I wish it was my sole) web browser.
Unfortunatelly, as expected, email services changed and some became broken.
Luckily jeroen kept updating scripts which allowed this extension to remain useful in PM (it lost a little recently because of Gmail becoming mostly broken, but I still prefer count it as a daily essential).
Same request
Join with the same request Palemoon is a good browser which continued to support old extensions like x-notifier, but the last 29.2 version stopped