Do ordinary web pages print normally or do they also suffer from the same problem?

With web pages:

If Firefox is not using the whole sheet of paper, this can be caused by having extracted the wrong paper size settings from Windows when reading the print driver data. Clearing it can involve finding some obscure settings, but here goes:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste print and pause while the list is filtered

(3) For each setting for the problem printer, right-click and Reset it. The fastest way is to right-click with the mouse and then press the r key on the keyboard with your other hand.

Note: In a couple other threads involving Brother printers, the preference printer_printer_name.print_paper_data was set to 256 and when the user edited it to 1 that fixed the paper size problem. If you see a 256 there, you can edit the value by doubling-clicking it or using right-click>Modify.

Any improvement?

There are other things that need attention.

Your System Details list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts. The user.js file will only be present if you or other software has created this file and normally won't be present.

You can check its content with a plain text editor (right-click: "Open with"; do not double-click). The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can delete the user.js file if you didn't create this file yourself.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Windows hides some file extensions by default. Among them .html and .ini and .js and .txt, so you may only see a file name without file extension. You can see the real file type (file extension) in the properties of the file via the right-click context menu in Windows Explorer.