I have resolved the problem. The 'Features' tab for the printer set the resizing option to 4x6. I changed that setting and the printer is working normally now. Previously Firefox would crash each time I selected the 'Features' tab of the printer settings.

I originally wrote: I cannot get my Firefox to print using the letter sized paper in the main paper tray. It will only print on the 4x6 photo stock. I have not had this problem before. Other programs (MS-Word) are printing correctly. I am using a Toshiba Laptop, printing on a HP Photosmart C5100 over a wireless network. I am running Firefox 3.6.10. I have tried deleting and reinstalling the printer through Control Panel. I have rebooted the computer and the printer. I have gone through all the settings I can find on Firefox. If I select File/Print and select the Properties of my printer, it indicated that I have selected letter sized paper from the main tray. I am not aware of changing any settings that would cause Firefox to print using the photopaper tray. The last time I used the photopaper tray was either with another program (Powerpoint) or, possibly, when I tried to print a picture from Facebook. Any help would be appreciated. (ps - I checked 'the other browser' and it is printing correctly.)