You can change the color on text pages with an extension like this one:

• NoSquint - https://addons.mozilla.org/firefox/addon/2592

Browser interface:

• Theme Font & Size Changer: https://addons.mozilla.org/firefox/addon/162063/

To make the text on web pages bold by default you will probably have to use code in userContent.css or use the Stylish extension as I'm not aware of an extension with that feature.

Add code to userContent.css

html, body, body * { font-weight: bold !important; }

See:

• http://kb.mozillazine.org/Editing_configuration
• Stylish: https://addons.mozilla.org/firefox/addon/2108

Another option: you can go into Options / Content / Fonts & Colors and change your "Default Font" to 'Arial Black'. You can also play with the font size there.

NOTE: just be sure that you click on the 'Advanced' button there and uncheck "Allow pages to choose their own fonts, instead of my selections" of you won't see any changes. You will need to do this first.

EDIT: The add-on mentioned in the above answer is very good quick-click way of doing the same thing I outlined.

Theme Font & Size Changer: https://addons.mozilla.org/firefox/addon/162063/