Mozilla Firefox is a fully customizable web browser. You can customize its appearance, such as color and themes, to suit your tastes. One of the features that you can customize in Firefox is the default font. Most browsers default to Times New Roman. In case you get bored or just want to make things more vivid when surfing, you could change the default font.
Steps
Method 1 of 2: Change the font for the pages
This will only change the font for pages and sites that do not have a predefined font.

Step 1. Open Mozilla Firefox
Double-click the shortcut icon on your desktop to launch the browser.

Step 2. Click the Menu button
This button is located in the upper left corner of the screen.
- For older versions of Firefox, click Tools in the menu toolbar at the top of the window.

Step 3. Select Options
This will bring up the window Options.

Step 4. Click Content
There you will find different options to change the way the fonts are displayed on the browser.

Step 5. Click the Default Font drop-down box
This will bring up a list of fonts you can choose from. Just scroll down and select the font you want.
If you want to change the fonts for all sites, continue to the next step

Step 6. Save the changes
Just click on OK.
Method 2 of 2: Change the fonts for all web pages

Step 1. Click the Advanced button
After choosing the font you want, don't save yet. Click on the button Advanced next to the button Font size. This will open the advanced font options.

Step 2. Uncheck the Allow pages to choose their own fonts option, instead of my selections above.
The sites have their own predefined fonts. If you do not uncheck this setting, the default font you predefined will only apply to pages and sites that do not have a predefined font

Step 3. Save your changes
Click on OK to save changes to advanced settings. Once again in the tab Contents, click on OK to save any changes you have made.