Zoom Page zooms in and out of web pages using +/- buttons and % display on any tool bar. Choose from four zoom modes: Default Zoom, Fit-To-Width, Default Zoom + Site Specific or Fit-To-Width + Site Specific. Button style and zoom levels are fully configurable.
Here is more about Zoom Page from the add-on developers:
Zoom Page allows a web page or image to be viewed larger or smaller than normal, using a multi-function button that can be placed on any tool bar. The current zoom level is displayed as a percentage and the zoom level can be manually changed using two buttons (plus & minus), or using the popup zoom menu, or using Firefox’s standard zoom shortcuts.
You can pick up Zoom Page from the Firefox Add-ons website.