I narrowed it down to this addon causing the problem. The play button doesn't work like it should (stays on play symbol) and the media does not start playing.
It may be a combination of my addons, I'm not sure. Can anyone else test?
First of all, I was with version 3 something, it updated to 4.0.2 without me realizing the need of exporting my settings... there I am starting again with a brand new x-notifier where I have to input my email addresses 1 by 1. Not a big deal I thought. I started adding 2 email addresses email1@gmail.com and email2@gmail.com to x-notifier.
The text of XN's toolbar button menu is "hardcoded" to black instead of theme's (default) menu item text colour. This results in illisible black text on black/dark background with black/dark themes. The bug is present in both 4.0.2 and Neo 2.1.1.
Additionally, the same issue is present in Neo's config/options page: the text colour is set to black, instead of theme's window text colour, again resulting in invisible text when dark themes are used in FFox.
When I wake up I usually see 10+ "new mail" count on X-notifier 3.5.23. This morning, it was zero. Seemed strange so I opened Gmail manually and found 10+ mails I had never read were marked as "already read".
I don't think this is Gmail side issue (if so, much more people claim the issue) so guess X-notifier 3.5.23 did this. I'm only using the Gmail account with X-notifier 3.5.23 on Cyberfox 50.0.2.