For my own reference: I've posted the problem on 14:37 PM 2021/08/10 UTC +1

It isn't clear to me what you mean. You have a US keyboard so the key below Esc show ` and the key to the left of Enter shows '. Your system uses a British keyboard layout so you need to use different keys to type those symbols. Does that mean you want to use the US layout on Firefox? I have never seen any reference to a way of changing the layout using Firefox. I have two (alternative) layouts on my system and the system provides methods for changing from one to the other, one an icon in the system tray, the other a keyboard shortcut. Perhaps your system provides something similar?

Do you have multiple keyboard layouts installed in Windows?

It is possible you accidentally changed the keyboard layout by using a key sequence to rotate the keyboard layout or otherwise changed the layout. It is usually best to disable the rotate layout feature and instead set a specific keyboard shortcut for each layout. Note that Windows remembers the keyboard layout setting per application.

• https://support.microsoft.com/en-us/windows/manage-the-input-and-display-language-settings-in-windows-10-12a10cb4-8626-9b77-0ccb-5013e0c7c7a2

My problem is solved. I've been stupid and chose the wrong setting on Windows. *facepalm*

I had the wrong keyboard layout selected as my standard on Windows.

Go to Settings -> choose devices -> choose typing -> scoll down and click on the lowest link 'Advanced keyboard settings'. Then check out which layout (in the selection bar) works the best for you, I suggest to choose one at the time and experiment (or just knowing which one you need, like I did not).

The only 2 questions left to me: - Is there a similar option on Firefox (in case Firefox does mess with this keyboard layout)? and - Where to find such an option?

Yes I did change the language with the hotkeys first (I think)