message-box font isn't what we're seeing there; the bulk of the about:memory content is inside
<pre class="entries">, and html.css sets
font-family: monospace for
So the question, then, is what font family is configured as
monospace for the locale involved, whether it really is monospaced, and whether it supports the box-drawing characters.
In English with default Firefox prefs, the default monospace font is Courier; as it happens, this doesn't support the box-drawing characters, but the font prefs also include Courier New as a fallback, and it does have these characters -- and fortunately, it has the same character widths as Courier, so the resulting mixture of fonts doesn't disrupt alignment.
However, if I set
lang="ja" on the
<pre> element in about:memory, the default monospace font is Osaka-mono (note: this depends on having the Osaka fonts activated in Font Book; they're not present by default on an English system, but are downloaded if you explicitly ask Font Book to enable them). The problem then arises because Osaka-mono is not truly "monospaced": it has Latin letters that are exactly half the width of its Japanese Kanji/Kana glyphs. (This is usually a good thing, because full-width Latin letters used for any significant quantity of text look really ugly.) The trouble is that the box-drawing glyphs in Osaka-mono have (quite reasonably) the same "full" monospaced width as Japanese characters, and therefore are twice the width of its Latin letters/digits.
A similar issue occurs with Chinese locales, by the look of it.
Probably the simplest fix would be to explicitly tag the language of the about:memory content with
lang="en", so that English font prefs are used to resolve the
font-family: monospace property on the
(Do we localize about:memory in localized Firefox builds? If so, that may complicate things a bit; but the screenshot here, at least, shows English content for which
lang="en" would be entirely appropriate.)