[Affected platforms:]
Ubuntu 20
Windows 10

[Affected versions:]
Nightly 89.0a1

[Description:]
When you access a submenu with the mouse and then come back from that submenu with an arrow key, the submenu option that you left is shown with the colored solid border on the main menu, as if you had gone down to that option with the arrow keys. But since that is not the case you can't use the keyboard. So, the problem is that it shows the border, but you can't use the keyboard.

[Steps:]

1. Open Firefox with a new profile.
2. Open the hamburger menu (or a toolbar menu)
3. With the mouse, click on a submenu like help
4. Once on the help submenu, press the LEFT arrow to RETURN to the hamburger menu again.

[Actual result:]
You can't use the right arrow to access the help menu again even though you have the orange border as if you had gotten there using the keyboard.

[Expected result:]
The orange border should not be over the help option nor shown since I accessed through the mouse.

Hi, this is a bit improved in Beta. Now the problem is present only if I've used the up or arrow key at least once before clicking the sub menu.

This is fixed on Beta 89.0b12