No subpixel font rendering in Firefox

## Current behaviour

In Firefox, all the text in Hackmd documents is rendered without subpixel antialiasing.


The text is hard to read on non-hi-DPI screens.

## Expected behaviour

The text should be rendered as set up in the system.

## Notes

It must be something to do with CSS styling. Possibly, semi-transparent backgrounds.

If I apply a user style like this
``` css
.CodeMirror-sizer { background: white; }
```
the code pane starts to render the fonts correctly:


## Environment

I use HackMD on:

** Desktop**
- OS:
- Arch Linux, KDE Plasma, X11
- Windows 10 Pro 22H2 19045.4651
- Browser: Firefox
- Browser Version: 128.0

In Chromium, everything is rendered correctly, with subpixel antialiasing.