Inexplicable display anomalies often indicate some incompatibility between Firefox and your graphics card driver software. Sometimes you can update or roll back that software to solve the problem. However, as an immediate workaround, try disabling Firefox's use of hardware acceleration for graphics.

orange Firefox button (or Tools menu) > Options > Advanced

On the "General" mini-tab, uncheck the box for "Use hardware acceleration when available"

This will take effect after you exit and restart Firefox.

Does that resolve the issue? If so, a compatibility issue is the likely problem.

If the problem only affects PDF documents, it could be that Firefox's built-in PDF Viewer is not rendering them with the correct font metrics. You can disable the built-in viewer and use the Adobe plugin, or another plugin, or the Adobe Reader application, instead.

This article has the steps for that: View PDF files using Firefox’s built-in viewer.

However, it seems you do not have any PDF reader plugins installed or maybe they are disabled. You can check for disabled plugins on the Add-ons page. Either:

• Ctrl+Shift+a
• orange Firefox button (or Tools menu) > Add-ons

In the left column, click Plugins. The disabled ones (set to "Never Activate") are grouped at the bottom on a gray background.

If your Adobe PDF plugin is older than 11.0.0.5 or 10.1.8, you may need to update it. For that, I recommend using the Help menu in your Adobe product, where there is a "Check for Updates" item.

thats helped thank you very much for all your help.:)