I have two phones with Firefox for Android version 85.1.3 installed on them. One phone has the Adobe PDF viewer also installed on it (the only PDF viewer on that phone), and it works fine whether I click on a PDF in a web page, or if I download the PDF and then open it from the browser. This phone has Android 7 on it. The other phone has a few PDF viewers installed on it (OneDrive, GoogleDrive, and something else). It is running Android 9. If I click on a link in a web page, I get a dialog that asks if I want to open in an app. If I say I do, it always opens in GoogleDrive. If I long-click on the link and choose to download it, then if I ask it to open, it still uses GoogleDrive. If I us a file-browsing app on my phone in the download directory, it gives me a choice of installed viewers, and I can choose one. So I did some exploration. I went to my settings app, chose Apps, then chose Default Apps from the triple dot menu. From there, I chose "Set as Default" which lists all my apps. I found "Drive", clicked on it, and pushed the Clear Defaults button. Now, when I go into Firefox and look in the download folder, and click on a PDF, it gives me the ability to choose which action I want to use. Whatever app I choose becomes the new default. It did not allow me to choose an app as a "just once" choice. So I would recommend you go into your settings app and look at default settings. Something in there must be causing your problem.