Where to find Options > Applications, find Portable Document Format (PDF), and set the action to Use Adobe Acrobat (in Firefox).

It has not been Tools > Options for a long time as it is Settings > General and scroll down to Applications

https://support.mozilla.org/en-US/kb/firefox-options-preferences-and-settings

Settings - General - Applications - change the "Open in Firefox" option via dropdown https://support.mozilla.org/en-US/kb/choose-your-preferred-pdf-viewer-firefox

Thank you. Only problem is that my version of Firefox does not show the General - Applications - change the "Open in Firefox" option via dropdown in Settings and I couldn't find something similar.

But I appreciate your help!

Maybe you can try:

1. menu - settings - find in settings - input pdf

If the above is empty, try

2. your file explore - select a pdf file - context menu - choose open with - select your application - set always

In your screen, I see Android 15. Mine is Android 14. I suggest that you file a bug for it. https://bugzilla.mozilla.org/