Print Preview Bug When No Printer Installed in FireFox



Before a user prints a page from the Internet in Mozilla FireFox, s/he can have the program display a 'print preview', a function that shows how that Internet page will print. However, if no printer is currently installed, FireFox encounters an error when launching the Print Preview. Error messages display informing the user of an "Unknown Error" and the Print Preview controls cease to respond, which interrupts (but does not halt) the user's Internet browser.

Application Description

Mozilla's FireFox is an Internet browser available for Linux, MacOS, and Windows. At the time of writing this presentation, FireFox was still pre-1.0 (meaning that it was still a preview release for testing purposes).

Test Design

This example demonstrates the use of Stress testing. We use Stress testing to examine how a program reacts to adverse conditions, and how well it is able to recover or quit in these conditions if necessary.

In this presentation, we are testing the Print Preview function, but we do not have a printer installed. We will be "stressing" the application by asking for a function to be run without giving the function the necessary components.

Stress testing is a powerful testing technique in that we can see how programs fail (or rather, how well they fail). In this presentation, we are asking the program to format a page for our printer, yet we have no printer. We expect that the program should be able to either give us a warning about this and return to functioning as normal or else display the page using generic printer settings.

Performing the Test

  1. Open Mozilla FireFox, and type in the address bar:

  2. On the top horizontal menu, click on File->Print Preview.
  3. While opening the print preview, we receive this error:

  4. Click 'OK', and then try to close the print preview by clicking on 'Close':

  5. Print Preview will not close, so try some of the other options, like changing the scale from 30% to 200%:


The Print Preview function has now stopped responding entirely. Attempting to change pages, the page scale, or close the toolbox will all fail. The browser, however, has not stopped responding. The user is still able to work on the Internet, but they will be unable to close the Print Preview toolbar.

This bug is not critical and does not affect the program in any serious way. The program is not damaged any further than to cause the user to close the window and open a new window to get rid of the Print Preview toolbar.

How likely is a user without a printer to use the Print Preview function without having a printer? The Print Preview function is excellent for web developers who simply want to see how a page would look printing, but do not necessarily have a printer. Similarly, the average user may simply want to see how something might print later, and be scared off because of the error message. Though the bug is of notable concern, the error message is equally important. The user may not know that there is not a printer connected, and because of the error message, just assume that the error is with the browser. In addition, the error message tells the user that an unknown error occurred "while printing." Does this mean that it was not actually trying to display a printable preview of the page, but rather was attempting to print the page?

Stress testing allowed us to quickly and easily find this bug because we simply targeted one function that we knew depended on a certain environmental condition, and removed that condition. From there, we directly observe how the program reacts.

Similar Tests/Additional Notes

Try this test in other browsers or applications where printing is an option without having a printer connected. Observe both how the program responds and what error messages you receive.

Configuration Notes

Testing Mozilla's FireFox v0.8 on:

Created 31 May 2004 for the CSTER

All images and written material ©Copyright Sam Oswald 2004

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