Does it happen on Nightly 32bit and Nightly 64bit on Windows 7 and Nightly 32bit and Nightly 64 on Windows 10?

I might suggest to reset the printer settings in the browser as the fist troubleshooting step for the affected computers.

• Fix printing problems in Firefox

No, that is exactly the point. It occurs only with Nightly i.e. the 64 bit browser and in both windows 7 and windows 10. It does not occur with Firefox, which I assume is 32 bit. Btw. I don't understand why you refer to Nightly 32! I thought Nightly is only 64 bit. Resetting the printer settings did not help. When trying to print the web page from Nightly I got the message "An error occured while printing". Changing the font had no effect either. The Firefox browser on the other hand had no problem printing the same web page to the same printer. Both browsers share the same user prefs.js profile!

The Nightly channel builds are available both as 32-bit and 64-bit on Windows.

In fact three of the four four channels (Release, beta, Aurora, Nightly) have both 32-bit and 64-bit for Windows except for Release as Firefox 42.0 Release may be the first to have Win64.

Linux and Mac OSX has 32bit and 64-bit on all four channels.

The Nightly channel builds is a bit of a work in progress at times as checkins on the channel (along with Aurora) happens almost every day.

If you want to be a Nightly channel Tester then see http://forums.mozillazine.org/viewforum.php?f=23 as many Testers hang out there vs here.

Thanks for the clarification. To avoid confusion, I have installed on my computer under Windows 10/64, Nightly 44.0a1 which figures under "Program Files" and should thus be 64 bit, as well as Firefox 41.0, which figures under "Program Files (x86)/Mozilla Firefox" and should be 32bit. The problem occurs only when trying to print a web page from Nightly, yet it worjs fine from Firefox. The symptoms are the same under windows 7.