I don't know about just the minimum size. But you can always use Ctrl+ and Ctrl- to scale the text sizes on a page up or down respectively. This should work in any browser incidentally. (though I think IE does a complete zoom on the page, not just scaling the text)

This may break a layout though. (text scaling) Well designed fluid layouts should work fine with text scaling, at least to a point.

You can also try View -> Toolbars -> Customize (or right click the toolbar -> Customize) and there you will see zoom icons to drop onto the toolbar. Though I think this is a complete page zoom, not just text scaling.

If you want to ONLY set the minimum font size higher or lower, you'll have to do so from Options. (this works the same for all web pages) If it isn't sufficient, try a higher minimum size. You can also set to use your chosen fonts and sizes and not allow website to set their own fonts, but then pages will look rather ugly. Last but not least, try the View -> Styles -> Basic Style option though most sites aren't designed for that. You will however be able to easily read the text. (it will use your default sizes and fonts that you have set in Options)

It is usually better to change the page zoom or text zoom than changing the minimum font. Zooming the page makes it possible for Firefox to adjust the size of the container as well and avoids disappearing or overlapping of text.

You can use one of these extensions to adjust the default font size and page zoom on web pages:

• Default FullZoom Level - https://addons.mozilla.org/firefox/addon/6965
• NoSquint - https://addons.mozilla.org/firefox/addon/2592