Refreshing Firefox should have cleared bad settings and reinstalling should have registered it with the system again. Good groundwork. Did you try Firefox's Safe Mode?

Have you tried reinstalling your printer? Do you use a wireless printer (notoriously finicky)?

Does the error prevent you from printing altogether or is it just a nag and you can print normally regardless?

Since upgrading to Firefox 43.0.4 I'm getting this message after printing one or two pages. I have to exit Firefox completely, and when I try to restart it, a message pops up saying it is already running, and would I like to stop it.

What's going on ???

To answer the latter first, the "already running" prompt results from the Firefox process not terminating properly. Even though Windows takes away the program icon from the taskbar, the process may keep running to finish up, and then ends up waiting for something, perhaps the printer?

If you need to end a process in Windows, use the Processes tab in Task Manager (to open, press Control + Shift + Escape).

I'll link the article, since this is another person's question: https://support.mozilla.org/en-US/kb/fix-printing-problems-firefox. Printer settings resets in Firefox seem to have helped in previous cases with printer issues (when done properly) and I've seen printer reinstalls fix issues when upgrading software.

Phoxuponyou (??)

I understood why Forefox wasn't restarting - I just added it as debug info to be complete.

I don't think your 'answer' in any way solves the problem. This printer issue started in Vers 43.0.4, so someone changed something and made printing unstable. I don't think I should have to go through a tedious "fix printer problem" exercise every time I start getting the message!!

No need to be rude or defensive. Might be the instructions solve it for good; I would have recommended the reset procedure regardless, the article just has a ready write-up for it.

43.0.4 release notes, https://www.mozilla.org/en-US/firefox/43.0.4/releasenotes/, say nothing of printing changes, so if this was due to a change in Firefox, it must have been an interim one between the version you upgraded from and 43.0.4. The OP had the issue with version 41.0 already it seems.

If you think that's rude or defensive, I'm sorry. It seems a bit factual to me.

sorry, please don't reuse old threads if you want to ask for help - open a new question instead. thank you for your understanding!