Running 52.0.2 on Linux here. I cannot reproduce the bug with those directions. If I hit / (which is on the same key as ? for me) and then hit ? (by use of Shift+/), then I see that behavior.
And I don’t really see it as a Discourse bug, myself. Although it would make it hard to search for question marks with the built-in Firefox page search.
Ah, I see, it is opening both the browser search and the Discourse keyboard shortcut similtaneously. Which is what I believe @Falco was seeing. You can barely catch it doing that in your screen capture though, but it is there (bottom left corner).