Hi Bob9754, Windows might have the wrong "file association" for .pdf files. Do you have a PDF viewing program such as Adobe Acrobat Reader that you can assign to PDF files instead of trying to launch them into Firefox? See whether that works as the first step. If Windows can't make the switch, there might be a problem with those settings in Windows, but I'm not very familiar with how to clean that up on Windows 10.

Thank you for your reply jscher2000. When I woke up the PC this morning I opened my Adobe XI reader app as you suggested and was able to access The .pdf files both through Windows file explorer as well as documents in my sys tray. I got a MS pop up "user account control" window asking "do you want want to allow this app to make changes to your device", the app being "Adobe Acrobat Reader Win_binaries". After clicking yes the file will then open in Acrobat reader without any problem. I am wondering if over the past few months perhaps MS in one of their updates which automatically down load to my PC has changed the way they handle Firefox .pdf files in their OS and this triggered the XPCOM error when trying to open them in windows 10. Once I opened the Adobe app, thereafter I could open all .pdf files in their OS using file explorer and sys tray although I get the user account pop up window for each file. I can still access these .pdf files from Firefox "menu" under "open file", which opens a Firefox file explorer window to open any of the .pdf files in documents. Theses do not open in Adobe reader, but open immediately in the Firefox tab I was using when I opened "menu". Prior to the XPCOM error I recently encountered with the .pdf files in Windows file explorer and sys tray, these files used to open in Firefox the same as it does when opening them presently from "menu" in a browser tab. I hope this helps others who may be experiencing the same problem. I can't say how long this has been happening as I haven't used a .pdf file since April of 2018.

Since your reader is free did you try to uninstall it and download the latest free version of XI reader and install it to see if anything changes?

I am using the latest free version of Acrobat that was installed as I get updates. .pdf files in Windows file explorer and sys tray now load without the XPCOM error, but each file loads only after clicking yes to the MS pop up "user account control" window asking you to allow it to make changes for the Adobe app. If I open the files using Firefox browser menu "open file" I can directly access any .pdf file with in the Firefox file explorer without using Acrobat. I used to be able to do this in the Windows file explorer and sys tray documents without getting the XPCOM error or being asked for app permission. I believe MS changed something. Bottom line is I am now able to use .pdf files from either the Windows OS or the Firefox browser, just a slight format change. It used to be easier to just open documents in sys tray and click on the ,pdf file and have open in Firefox without getting the MS pop up "user account control" window asking you to allow it to make changes for the Adobe app.