After installing Adobe Acrobat, can I revert to Preview for handling PDF from Firefox?
1st thing to try

Open Firefox
Open Preferences (Firefox &gt; Preferences, or Command + ,)
Select the Applications tab
Search for &quot;pdf&quot;
Change action to something else, then change it back to &quot;Always Ask&quot;


Verify this is set correctly

Open Finder
Search for .pdf
Right (Option) Click the first PDF that appears.
Click Get Info
Expand Open With (if it isn't already)
Change it to &quot;Preview&quot;
Click Change All button below the drop down menu
Accept the prompt.

For a visual representation: