No fixed width fonts in GMail?

I rarely use GMail, so I didn't care to look into this so far.  I don't care much today either, but I sent a plain text message to someone, and I carefully formatted it in Emacs, and I know that this someone is using GMail and that by default GMail displays text messages in a variable-width font, so I thought — let's check to see if there is an easy way to display text messages the way they should be, that is, in a mono spaced font.

Looks like there was a way, but not anymore.  Now you have to resort to all sorts of hacks, like using GreaseMonkey and writing a custom CSS for GMail.  How to explain this to someone I write to, or how could I expect anyone I write to to be able to master Firefox so well that they are able to customize GMail by writing CSS?

I also found a Firefox extension (which is too much as well to ask from recipients, but just wanted to look into it).  But the last release was sometime in 2008 and it no longer works with the current Firefox.

Why is there so much silence about it, and why is there no obvious reason why they would remove such a simple feature?  Any other email client on the planet knows that plain text should be displayed in a fixed font.

I wonder if the real reason has to do with the fact that the default font makes all emails look like Adsense :-(