Firefox doesn't use the System default printer as its default printer. In Firefox the "last used" printer is used as the "default printer".

If you're on a network, Firefox may not remember the last used as the default all the time. Laserjet printers that have gone to "sleep" or into energy-saving mode seem to be problematic as being remembered as the "last used", even in non-network situations, when you have multiple printers connected.

Thank you. However, I am not on a network and I never had this problem before. Will I now always have to remember to check the printer setting every time I wish to print out an email in order to correct the printer setting? Why would I have this problem now when nothing has changed? I appreciate your response.

Firefox should remember the last used printer from one session to the next. The only time you should need to select a printer is if you want to use a different printer than the last time you printed something.

If that selection isn't be remembered, try the reset described here:
http://kb.mozillazine.org/Problems_printing_web_pages#Reset_printer

Make sure you reset all the prefs for that printer.