I came across a great add-on for the Firefox web browser. It is called NoSquint. It enlarges the text and pictures on web pages which makes it very easy to read. While there already existed in Firefox the ability to zoom in on the text with the key combination Ctrl-+, this is much easier.
To get it, from within Firefox select from the menu: Tools->Add-Ons->Get Add-Ons. In the search box that is displayed type in nosquint. The nosquint plugin will show up in the list. Just select it and hit the install button.
After you have installed it, to adjust the default zoom level go to: Tools->Add-Ons->Extensions. Click on the nosquint plugin and then on the preferences button. There you can adjust the preferences.
See the following for more information and screenshots.