Windows 7: Firefox refuses to open pdf's and just wants to save them

Hello,
Using Firefox, I normally open online pdf's with PDF-XChange Editor. This doesn't work anymore. Now, when I click on the pdf link, FF only offers to save the file, even though my Application Preference for pdf's (in about:preferences) is still set to "Use PDF-XChange Editor". This issue seems to have started after a friend of mine used my machine to open his webmail and downloaded a pdf. I have restored my system to a previous date with no success, so it may seem that the registry was not modified. I also tried to set my Application Preference to use another software able to open pdf's, but FF still wants to save the file. Finally, I do not have the option to open pdf's within FF, but if there was a way to do it, I'd take it.
I cannot reinstall FF as I use an older version (61.0.2; this is because I absolutely need the Properties/Descriptions of my bookmarks, which have disappeared in recent versions of FF).
I am attaching the pdf-related items from my about:config in case this would help spot the culprit.
Otherwise, I am desperate as sometimes I may have to read 10-20 pdf's a day, and I can't afford to save/delete those files every time. So, if you have any suggestions please do not hesitate to post them
Thank you!
PS: I don't know if this is relevant but after my friend's access to his webmail on my machine, my default printer was set to "Send To OneNote", which I of course changed back to my usual printer.

jmm,
I use Firefox exclusively.
Your setting in Options ->Applications for Firefox PDF is the problem.
It's set to Save File!
See my OneDrive >here<



Bill

How could I miss that?! I only checked pdf, not portable document format...

Thank you so much Snick!!

Extra minds, extra thoughts, extra ideas, extra viable solutions!
Happy Computing.
KISS
Keep It Simple Sir

Bill