No longer tolerate unreadable text! Set a minimum font size for all web content using a toolbar button.
Here is a little more about this add-on from the developers:
The No Small Text toolbar panel enables you to set a minimum font size for all web content.
This minimum font size may be enabled or disabled by clicking the toolbar button, which will show a green or gray icon, respectively.
Some users, such as translators or multilinguals, may find it useful to set differential sizes for different font groups using the sub-menu.
No Small Text is compatible with NoSquint and similar add-ons and may be used to extend their functionality.
Want to learn more? You can pick up No Small Text from the Firefox Add-ons website.