I've just discovered, that by typing text into urlbar and then clicking enter while holding SHIFT or ALT leads to a different behavior by opening tab in new window or new tab respectively.

But I have custom layout with caps+space acting as enter via xkb 8-level mapping, and that enter always opens new tabs. I suspect what is happening is that firefox thinks that I have alt down when the keyboard state is in third level. But I don't really use those modifiers so is it possible to just disable this behavior and open everything in same tab?