After some research I've realized that the `centerscreen` feature triggers this problem. On Google Chrome `window.create()` accepts `left` and `top` for popup type windows, and omitted coordinates are filled with "(`left` of the last normal window + 10, `top` of the last normal window + 10)". So my patch does same thing on Firefox, except applying `centerscreen` if both `left` and `top` are omitted.
Bug 1271047 Comment 11 Edit History
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After some research I've realized that the `centerscreen` feature triggers this problem. On Google Chrome `windows.create()` accepts `left` and `top` for popup type windows, and omitted coordinates are filled with "(`left` of the last normal window + 10, `top` of the last normal window + 10)". So my patch does same thing on Firefox, except applying `centerscreen` if both `left` and `top` are omitted.