I'm new here, and I'm going to add my vote to the "New Font is an Abomination" pile. Worse is the attitude of fastmail staff - suck it up. That's what I was told:
"Thank you for taking the time to provide this feedback! I'm sorry to hear you don't like our latest update to the UI.
I have passed your feedback along to the department concerned. Unfortunately, it is not possible to revert to the old font, sorry 
If you have any further questions or problems, please feel free to contact me. "
I reminded them that it absolutely IS possible to revert; all they need to do is re-enable that option. What does it cost them to keep the option to retain the old font? Nothing. I suddenly feel stupid for paying for a service that doesn't let me pick my fonts. That really should be a basic minimum option.
It's possible to set your browser to deny websites the ability to select fonts; that helps a little, though may mess up other sites. In firefox, there's also the add-on "Font Blocker" which allows you the ability to right click on a font you don't like, and set the add-on to deny the website access to use the font. It's imperfect, but a start. These are the work-arounds that we're stuck with thanks to the attitude of fastmail. They must be rolling in users and cash that they can take such an inconsiderate, "our way or the highway" attitude to their users.
Interface changes will be plagued with issues of differing user tastes. Deal with it, fastmail. Your users are what make you. Don't tell them to suck it up. Give us options.
I've been looking at protonmail for some time for numerous reasons; this regression may be the thing that forces the migration.
Thanks for enduring my rant.
