Some websites buttons that opens a menu won't close after clicking on the button again
The best example is Youtube's notification button (bell). When you click on it it opens the menu just fine, but when clicking the button again to close it, it just reloads it. The only way to close the menu is to click somewhere else on the page, which I often do in the video section and will register my click as a main action and pause the video. It's a bummer, happen to me every day. On other Browsers, clicking on the button when the menu is open just close the menu without affecting the browsing.
Opaite Mbohovái (4)
I'm not sure about any other websites that you may be having issues with, but for your example of the YouTube notification icon, that's just a design flaw with the website. Unfortunately, reloading the notifications by clicking the bell icon is the intended behavior and it occurs across all browsers, including Google Chrome.
In most cases, website issues aren't anything to do with Firefox. Instead, they are usually an issue with the way that the website has been designed. The team over at Webcompat aim to diagnose and notify websites of issues with browser compatibility. If you come across an issue with a website that is only with a specific browser, reporting it to the Webcompat team normally yields the best results.
On Mac, does the Esc key close the notifications panel?
Thank you Wesley, your contribution made me perform tests on other browsers and confirmed what you said. I think it just happened that this Youtube function changed around the same time I switched back to Firefox two months ago. What a bummer. I will notify this to Webcompat instead as you suggested. Thanks for your help and if you work in IT I hope you have the position you deserve: a really good one :)