Hi,

Please also try to Disable Adobe PDF in Firefox Tools (Alt + T) > Add-ons > Plugins, and instead right-click and Toggle to false the key pdfjs.disabled in about:config to enable the built-in PDF viewer.

1. In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear.
2. Click I'll be careful, I promise!, to continue to the about:config page.

about:config Entries

The Config Descriptions add-on adds helpful source comments in about:config.

Viewing PDF

Using extensions

Which plugin version are you using for PDFs? For example, this is what I see:

orange Firefox button or classic Tools menu > Add-ons > Plugins category

Adobe Acrobat 10.1.4.38

If the problem occurs on some particular websites, could you provide a URL or describe any reason those might be different from other PDFs?

If the problem occurs for all PDFs, could you test those sites in Safe Mode? That's a standard diagnostic to bypass interference by extensions (and some custom settings).

First, I recommend backing up your Firefox settings in case something goes wrong. See Back up and restore information in Firefox profiles. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, restart Firefox in Firefox's Safe Mode (Diagnose Firefox issues using Troubleshoot Mode) using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode."

If PDFs works normally, this points to one of your extensions or custom settings as the problem.

Any difference?