Yes, they are easier to design, and easier to parameterize, which is nice for you, the designer. But they are not nice for me, the reader. Using a sans font says to me that your comfort is more important than mine, and that your time is worth more than mine and everyone else's, combined.
In Firefox, I have disabled web fonts, and use Libertine everywhere. That choice is denied to me in Firefox on my phone.
I don't quite understand your criticism of sans-serif fonts.
However I wanted to address your last sentence. Perhaps you could try to see if the "Stylus" add-on works on Firefox mobile. I haven't tried it myself but it should.
Yes, they are easier to design, and easier to parameterize, which is nice for you, the designer. But they are not nice for me, the reader. Using a sans font says to me that your comfort is more important than mine, and that your time is worth more than mine and everyone else's, combined.
In Firefox, I have disabled web fonts, and use Libertine everywhere. That choice is denied to me in Firefox on my phone.