In Preferences choose Appearance, and under Apearance you will find Font. This will bring you to a similar font selection screen. The Encoding pull-down menu, you will see, has several choices, one for each language Netscape supports. This indicates the width of the individual characters within the font. Some fonts, like Courier, emulate typewritten text, and each character takes up the same amount of space in the line.
Proportional fonts are more like professionally typeset text.
Most font descriptions will tell you whether a font is fixed width or proportional. Unfortunately, you'll have to remember which is which as you scroll down the enormous list of fonts on your system. If you're like me, you'll have trouble even remembering the names of the fonts you've just installed, let alone whether they're proportional or fixed. Jotting this stuff down before you get involved is a good idea if your mind is more like a sieve than an elephant's. Get all these right, and you'll be ahead of the game.