Minimum Font Size doesn't work
I've set Tools > Options > Content : Fonts & Colors > Advanced > Minimum Font Size to 10 pt, but when I use Zoom Out, I get smaller fonts: The relation between font sizes on a page is kept. What I want is FF 1.5 behavior: Minimum Font Size is honored even on Zoom Out, which results in shrinking the headlines only and finally ending with all Text in the same size after some repetitions of Zoom Out. I have disabled Tools > Options > Content : Fonts & Colors > Advanced > "Allow pages to choose their own fonts, instead of my selections above". I have set Minimum Font Size for several encodings, but maybe the reason for the current behavior is a missing UTF-8 setting in the Tools > Options > Content : Fonts & Colors > Advanced > "Fonts for" listbox ? Most pages including this support.mozilla.com use that encoding.
Opaite Mbohovái (2)
If you want headings to appear at the same font size as the text then you can do that with code in userContent.css or Stylish.
H1,H2,H3,H4,H5,H6 {font-size: 100% !important;}
See http://kb.mozillazine.org/Editing_configuration#How_to_edit_configuration_files
Main Menu -> View -> Zoom -> Zoom Text Only