If you want to simply change the default font from Times New Roman to your default Sans-Serif font, Arial, do the following:
1. Open up your favorite text editor. Notepad will do.
2. Copy the following line into a new file:
3. Save the file as:
to your *.slt directory located here:
This link will tell you where that directory is located for your OS:
Now, the next time that K-Meleon starts, this preference is copied over into your prefs.js file, which is located in the same directory. Technically, you could add that line to prefs.js but by locating it in user.js, you don't have to worry about it getting hosed if the prefs.js file is overwritten. If you have done this properly, your default font should be your Sans-Serif font [CSS might override that in some cases - not sure].
If instead, you want to change your default Serif font from Times New Roman to say, Georgia, add the following line instead:
I hope that helps.