Crypto warning popup is not screen reader accessible

There are a few problems with the warning popup that was implemented for #40209 (closed) when using a screen reader. They boil down to:

• Not being semantic. E.g. using <xul:description> and <xul:label>.
• Not moving focus into the popup, even though it requires a response. This means you cannot navigate it with a screen reader in "browse" mode (Edit: you can move into the popup with F6).