Could you try reproducing this and after that see if the following also reproduces this by copying / pasting this into the browser console and pressing return. You will need the devtools.chrome.enabled pref set to true.

{
let action = {
label: "OK",
accessKey: "O",
callback: null,
};

let icon = "chrome://browser/skin/addons/addon-install-installed.svg";
let options = {
name: "Test",
message: "Test has been added to Firefox.",
popupIconURL: icon,
onDismissed: () => {
AppMenuNotifications.removeNotification("addon-installed");
resolve();
},
};

AppMenuNotifications.showNotification("addon-installed", action, null, options);
}

I haven't seen this problem for a very long time, so not sure if or how fast I would be able to give an answer to this question.

I would tend more to set this bug as WFM and reopen if I see it again.

If you see it again please try to also reproduce it using the steps in comment #8. Chances are that this is specific to the UI code and affects other notifications than just app update.

I just saw this again with a recent Firefox 73 beta build. The update notification was totally misplaced. As instructed I run the code from comment 8 and I can reproduce it in some way, but not that dramatically.

Here some steps:

1. Connect an external screen to your Mac, and make it the primary one (mine is on the right side)
2. Open the Browser Console and paste the code from comment 8
3. Start Firefox and move the window to the secondary one, so that the hamburger menu is actually not visible (maybe there is another GFX bug but unless I move the Firefox window somewhat >50% over to the external screen nothing of the window appears there)
4. Execute the code in the Browser console
5. Click and move the browser window to the left until it is completely shown on the internal screen (left side)
6. The notification has been made visible and is partially off, or even shows two arrows at the top