UPDATE (still no solution):

Found how to reset changed print settings (see link below) - I did this, closed and restarted Firefox, no help.

https://support.mozilla.org/en-US/kb/fix-printing-problems-firefox

-jls

In the rare times I get an error it reads "Print Preview Error - An error occurred while printing". I have tried printing both with the Print Preview facility and the "Print Using the System Dialog" option. Sometimes I get the error above but most times I get no error but have to look at the print queue and see that the files are trying to print and always say "Spooling" but never actually complete.

Thanks much,

-jls

As of this writing, my computer has been up for nearly 3 days. I wish it were that easy, but this problem has been going on for months and my system has been rebooted many times. I normally keep my printers turned off until I need to print something. I close Firefox each night before sleep. All other programs can print at all times, far as I can tell, whether Firefox is executing or not. I experimented with all that early on, including rebooting the computer with the printers off, then turning the printers (I have two) back on and immediately trying to print from Firefox. Same failures as always. I have narrowed the issue to Firefox and nothing else. Something's going on such that, when the prints are stuck in the Spooler, exiting Firefox clears them. I know that has something to do with the problem, but I can't figure out why. BTW, even when there's Firefox prints stuck in the Spooler, other applications' prints come through fine. That's also trying to tell me something.

I appreciate your efforts very much! I'm still hoping a solution will present itself. Thanks much,

-jls

I've tried them both connected as Ethernet and WiFi. Same problems with Firefox. Both printers always print from any other application normally. It's definitely localized to Firefox. I have went so far as to completely uninstall Firefox (on Windows 10 newest update 64-bit) and reinstall from scratch. There may well be something going on in the Registry, so I uninstalled and installed Portable Firefox instead with no help. Still could be something in hidden settings somewhere, but I've tried everything I can think of.

Some additional background: I've used Firefox pretty much ever since it was Netscape, so 20 years+, so I'm not a newbie by any means. I hope this ends up to be something totally obvious that I'm missing. I wouldn't mind that one bit at this point.

Thanks as always,

-jls

Update (not solved): It occurred to me that the problem could be permissions, so I ran Firefox as Administrator to see if that had any effect. Unfortunately, same problem.

-jls

I always download software from the producing company unless they direct me to another site. I use IObit Uninstaller to thoroughly remove a program when the provided uninstaller doesn't do a full cleanup. I used Search Everything in this case to find and delete all traces of the previous installation and residual files. I cleaned everything as far as I possibly could. There obviously has to be something I missed, but I don't know where it would be; most likely the registry. I will try to find more with Regedit's search. It seems to me that when I went into about:config and did a reset on the print settings, I would have gotten the result I was looking for even if all the above failed.

Thanks,

-jls