When exporting a PDF file from Firefox to the DT Inbox shown in Finder the file is imported properly but the DT log shows a failure. When exporting a PDF file from Firefox to the Desktop and then dragging it to the Sorter’s Global Inbox, the file transfers properly and no Log item is generated.
Exporting a PDF how?
I just clicked the Save button in the PDF viewer in Firefox and saved it to the Global Inbox’s alias in the Finder with no issue.
Jim, I used the Print…Save as PDF in Firefox. Saved to Inbox. Have done this three times, same result each time.
Does DEVONthink have Full Disk Access enabled in System Settings > Privacy & Security?
Do you have tried this when DEVONthink is not running?
Maybe the printing of the web page is slowed down for some reason and when DT instantly tries to import the partial PDF file (or a temporary file which I have seen Firefox creating when it is downloading) that causes the failure.
The inbox is indeed not intended for slow processes link scanning or downloading. Printing should be usually fine.
That fixed it, thanks. Seems as if I needed to activate Full Disk Access for the beta after following your instructions for removing DT3.
You’re welcome 
Also, I highly recommend you read the Getting Started chapter in the Help.