Perform the suggestions mentioned in the following articles:

• How to print web pages in Firefox
• Fix printing problems in Firefox

Check and tell if its working.

May not be related to your problem but do the following:

-> Update Firefox to the latest version 10

• How to download and install Firefox on Mac

-> Update All your Firefox Plugins -> https://www.mozilla.org/en-US/plugincheck/

• When Downloading Plugins Update setup files, Remove Checkmark from Downloading other Optional Softwares with your Plugins (e.g. Toolbars, McAfee, Google Chrome, etc.)

One suggestion worked from the Firefox prints incorrectly link mentioned above by mha007. I'm thrilled since this has been annoying me for weeks. Thanks mha007!

Reset all Firefox printer settings

1. Open your profile folder:
2. On the menu bar, click on the Help menu and select Troubleshooting Information. The Troubleshooting Information tab will open.
3. Under the Application Basics section, click on Show in Finder. A window with your profile folder will open.
4. Note: If you are unable to open or use Firefox, follow the instructions in Finding your profile without opening Firefox.
5. On the menu bar, click on the Firefox menu and select Quit Firefox.
6. In your profile folder, copy the prefs.js file to another folder to make a backup of it.
7. Open the original prefs.js file in a text editor (such as TextEdit).
8. Remove all lines in prefs.js that start with print. and save the file.
9. If something goes wrong when you open Firefox, close it again and overwrite prefs.js with the backup you made.

Same problem on Firefox 13.0.1 on Mac 10.5.8, default font Verdana 16.

No problem when tested with Camino browser.

Did all of this and no change: https://support.mozilla.org/en-US/kb/Firefox%20prints%20incorrectly?s=printer&r=0&e=es&as=s

Reset all Firefox printer settings

If the above steps did not work, you may want to reset all of Firefox's printer settings:

   Open your profile folder:
       On the menu bar, click on the Help menu and select Troubleshooting Information. The Troubleshooting Information tab will open.
       Under the Application Basics section, click on Show in Finder. A window with your profile folder will open.
       Note: If you are unable to open or use Fire​fox, follow the instructions in Finding your profile without opening Firefox.

   On the menu bar, click on the Firefox menu and select Quit Firefox.
   In your profile folder, copy the prefs.js file to another folder to make a backup of it.
   Open the original prefs.js file in a text editor (such as TextEdit).
   Remove all lines in prefs.js that start with print. and save the file. 

This problem has occurred from time with different FF upgrades and then fixed itself with another upgrade.

I have also confirmed that all my Plugins are up to date.

The bizarre thing is that the greater the scale reduction, the larger the text in the pages following page 1. So, 75%, as in image attached, means that the scale for the pages after Page 1 increases more, and more text is lost, than if the scale is reduced to just 90%

There seem to be loads of threads reporting this problem!

Start Firefox in -> Diagnose Firefox issues using Troubleshoot Mode to check if your add-ons/extensions or hardware acceleration are causing the problems.

• Also see this article -> Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Check and tell if its printing normally while using Firefox in Diagnose Firefox issues using Troubleshoot Mode.