If you are using your pre-Windows 10 Firefox settings, I would try clearing print preferences. You can use the method described in this article: Fix printing problems in Firefox or the following:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste print_pr and pause while the list is filtered

(3) Right-click the print_printer preference and choose Reset to clear the previously saved default printer

(4) In the search box above the list, type or paste printer_ and pause while the list is filtered

(5) For each setting that specifically mentions the problem printer, right-click and Reset it. The fastest way is to right-click with the mouse and then press the r key on the keyboard with your other hand.

Any improvement?

Oops... I did something wrong. My search for printer_pr originally had 2 line items, now it has none.

printer_ search finds all line items beginning with "print.printer"

What now?

There's one that's print_printer which is the currently saved default which won't match the second search. Otherwise, you should be covered.

Then can you call up either print or print preview?

I have selected a default printer that now shows in print_printer but the print command still gives me the error msg "The selected printer could not be found."

This problem can be very trying, and I'm not sure in any given case what will work. In another recent thread, a Windows 10 user reported:

You'll never guess what fixed it: after I changed the default printer in Microsoft Edge it started printing okay in Firefox!!

https://support.mozilla.org/questions/1077400

On a suggestion I tried initiating printing from File->Print and got my usual print preview window and was able to print without problem. Then I tried my usual way of printing via Firefox->Print and it was suddenly working as usual (!). Nothing is ever easy with MS!

Matter resolved. Thanks, jscher2000.