1. Install the Keyconfig extension from this page:
http://kb.mozillazine.org/Keyconfig_extension
2. Make sure the Firefox Menu Bar is visible.
3. From the Tools Menu, select Keyconfig.
4. Click the Add a new key button.
5. In the Name fleld, type a name for the keyboard shortcut, such as "Spacebar."
6. To make this apply to all pages, click the Global checkbox.
7. Click OK.
8. In the Keyconfig main window, select the new shortcut, such as "Spacebar."
9. Click where it says <Disabled>, and type a space. The term <VK_SPACE> appears.
10. Click Apply. 
11. Click Close.

The question of how to disable spacebar scrolling is discussed on these pages: https://support.mozilla.org/en-US/questions/935601 http://forums.mozillazine.org/viewtopic.php?f=38&t=722865&start=0 The solution given is to use the Keyconfig extension to reassign the use of the spacebar. However, the instructions are somewhat incomplete. After finally getting it to work, I wanted to elaborate on the instructions. (Since those topics are closed, I'm starting a new one.) <pre>1. Install the Keyconfig extension from this page: http://kb.mozillazine.org/Keyconfig_extension 2. Make sure the Firefox Menu Bar is visible. 3. From the Tools Menu, select Keyconfig. 4. Click the Add a new key button. 5. In the Name fleld, type a name for the keyboard shortcut, such as "Spacebar." 6. To make this apply to all pages, click the Global checkbox. 7. Click OK. 8. In the Keyconfig main window, select the new shortcut, such as "Spacebar." 9. Click where it says <Disabled>, and type a space. The term <VK_SPACE> appears. 10. Click Apply. 11. Click Close.</pre>