Firefox and Chrome dark mode
I found many bad links about how to get dark mode in Firefox. The best way to make Firefox itself dark, is to open developer tools > hit the gear button and select the dark theme. To increase the font size within, just press Ctrl and "+" button. The Ctrl method increases the font size in Chrome as well. If you want to get a dark mode chrome, click on the ... (vertical) at top right corner of the developer tools > settings > Theme: Dark.