This comment on another UX.Stackexchange question:
that's not to force you to read it, it's to ensure that websites don't pop up the window as soon as your mouse pointer goes to click something in the same position as the install button.
reminded me of a problem I do sometimes encounter - I'm in the middle of quickly typing a text and all of a sudden some unrelated application considers itself important enough to not only pop up and visually steal focus (which is annoying enough) but also steals input focus such that keypresses that already left my brain and will be executed by my hands a few milliseconds (or even microseconds?) after the focus steal, effectively causing my just pushed N or ENTER or similar to trigger a function of that popup, which then vanishes before I even realize what happens and merely notice a few letters are missing from my text. Well, until my PC suddenly restarts since it was an automated updater...
So, while said comments states Mozilla products add a delay to the extension installation dialogue to mitigate this partly, is there any valid reason for focus stealing? (Note I am not referring to intended focus shift, e.g. when I hit the Windows key I do expect keyboard focus to switch to Windows' start menu; I am talking about a shift of focus that has no correlation to my current input)