How to Zoom In & Out With Firefox Browser
Like most Web browsers, Firefox 33 for Windows lets you optimize the appearance of websites with features like custom zoom levels and a minimum font size. While you can set both the zoom and minimum font size through the Firefox menu, there are keyboard shortcuts available so you can swiftly change your zoom level on the fly.
Like most tasks in Firefox, setting the zoom level involves opening the Firefox main menu, which is the button consisting of three horizontal lines in the upper right corner of the browser. Click the "Plus" icon to zoom in 10 percent and the "Minus" icon to zoom out 10 percent. Between the two icons is the zoom percentage; clicking the percentage will reset the zoom level to 100 percent. The maximum amount you can zoom in is 300 percent, while the minimum is 30 percent.
Using only the keyboard, you can hold the Control key and then press either the Plus or Minus keys to zoom in or out, beginning in increments of 10 percent to as large as 50 percent with each button press. Using the keyboard and mouse, you can hold the Control key and scroll forward or backward on your mouse wheel to zoom in or out in similar increments.
Minimum Font Size
To set a minimum font size for all websites, open the Firefox menu and click "Options," then click the "Content" tab. In the Fonts & Colors section, click "Advanced" and use the Minimum Font Size drop-down menu to set the minimum font size. You can set the size from as small as size 9 to as large as size 72. Uncheck the box next to "Allow pages to choose their own fonts, instead of my selections above" so individual websites do not override your selections. Click "OK" to save all changes.