Consider adding about:tor and others to the list of pages that do not need letterboxing
Currently about:tor is letterboxed when resized, despite being a built-in 'systemy' page (like about:preferences or about:config). We can remove the letterboxing for this page with this patch:
However, we may not want to do that and perhaps we want letterboxing for all content, regardless of whether it is a system page or not to avoid confusing users about different behavior.