further update posted here on Reddit https://www.reddit.com/r/firefox/comments/dmrb93/furstrated_with_firefox_70_et_al_useless/

Seems that this perhaps is a bug in Firefox when using alerts.useSystemBackend

make sure you are not always running firefox on your background , try after disabling it via task manager and click the notify msg again

Firefox needs to be running so that when a Action Center tile is clicked it can open the page being referenced. That is how Chrome handles notifications. If Chrome is running then clicking on a Tile in Action Center opens a tab in the already running Chrome and the YouTube page plays the video in question.

PROBLEM REASON:

It is a known bug or issue present in Windows 10 newer versions. Many people claim that the issue was resolved by a new cumulative update specifically KB4476976 released on January 22, 2019 but we have received reports from various Windows 10 users that they are still facing this issue.

Some temporary workarounds to fix this issue include restarting computer, logout and login again and restarting Windows Explorer. But these workarounds don't fix the problem permanently.

   PROBLEM SOLUTION:

This bug or issue occurs in Windows 10 devices when an option to let apps run in background is disabled. If a user has turned off this option manually in his Windows 10 computer, he'll start facing this problem related to Action Center and notifications.

To fix this issue, you just need to manually enable the option using following steps:

1. Open Start Menu and click on Settings option. It'll open Settings app. Alternatively, you can direct open Settings app using WIN+I hotkey.

2. Now click on "Privacy" item present in Settings app.

3. In left-side pane, scroll down to bottom and click on "Background apps" section.

PS: Alternatively, You can directly launch this page using "ms-settings:privacy-backgroundapps" command in RUN dialog box as mentioned here.

4. Now in right-side pane, enable "Let apps run in the background" option.

Let_Apps_Run_Background_Windows_10_Privacy_Settings.png

5. Restart your computer to take effects.

PS: If the above mentioned option is already enabled in your computer and you still face notifications problem, first disable the option and restart your computer. Then enable the option again and restart your computer. It'll fix the issue.

That's it. Now Action Center will start showing notifications in Windows 10.

That is already configured properly because Action Center is showing notifications from Firefox. Firefox is not in the list of apps on that screen anyways.

THAT IS NOT THE PROBLEM.

The problem is those notifications are not actionable i.e. clicking on the notification in the Action Center does nothing except making the notification tile disappear. Clicking on a notification say from YouTube should open the page being reference on YouTube.

IT DOES NOT.

If you click out side of the Action Center and then open the Action Center  again the notification tile that disappeared reappears in the list of notification.