On my x64 Win 7 system, practically the whole system was frozen. After the "Print" button was pressed the dialog stating "Preparing" came up, but I had to use Cntrl+Shift+Esc to open task manager. Then I had to use keyboard arrows to select firefox.exe and Alt+E to end task and the space bar to confirm. Mouse would move, but not "click" anything. If I attempted to click anything, I had to Cntrl+Alt+Del to get to where I could "Log Off" and that forced firefox to quit.

I found my problem was an addon with improper settings. Internet Download Manager addon was loading with firefox, but in IDM's actual options, the check box to integrate IDM with Mozilla was NOT checked and the Advanced integration with browsers was also unchecked. I had been having issues with downloading from sites that do not allow 3rd party software to download, and it was taking forever holding down the key that prevented IDM from intercepting the download, so I changed the option in IDM, and I could still right click and select "download with IDM" from the context menu. Re-selecting the check box next to Mozilla fixed the problem