In bug 1821353, we will add support for changing font to a font in the standard list. It is easier to implement because it doesn't require us to embed the font within the PDF.
As a next step, this bug will track implementing support for embedding fonts, allowing the usage of any font (e.g. OpenDyslexia).

Embedding the font is only one part of the problem, the second part is about the glyphs layout.
If you take "é", with some fonts we need to position the glyph for the accent and the glyph for the e.
We'd need to take into account the kerning: if we have the width for "VA" we won't have the correct one if we just put a V and then a A, the negative space between V and A must be taken into account.
And the ligatures...
And we don't want to write a glyph layout engine, it's a hard work and Firefox already has one so we should use it.
Something we could imagine is to send a chunk of text, with styles, fonts, ... to the pdf printer and then get back a pdf we could parse in order to extract what we want (see bug 1794809), this way the fonts will be subsetted and the glyphs will be positioned: everything we need.