To disable the default PDF viewer or plugin in the Firefox web browser, follow these steps:
Open the Firefox web browser.
In the address bar, type about:addons and press Enter. This will open the Add-ons Manager.
In the Add-ons Manager, click the "Plugins" tab.
In the list of plugins, look for an entry for "Portable Document Format (PDF)".
To disable the PDF viewer or plugin, click the "Disable" button next to the plugin.
Close the Add-ons Manager.
After disabling the PDF viewer or plugin, Firefox will no longer open PDF files in the browser. Instead, it will offer to download the PDF file or open it in a standalone PDF viewer application.