Did the page finish loading (i.e. you have the reload button present on the location bar)?

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Yes, the page had loaded fully. I can scroll down the email but won't stay - immediately it jumps to the top. I will try the above suggestions. THANKS!

You may have switched on caret browsing.

• http://kb.mozillazine.org/accessibility.browsewithcaret

You can toggle caret browsing off/on by pressing F7 (Mac: fn + F7).

• Options/Preferences -> Advanced -> General: Accessibility
  remove checkmark: [ ] "Always use the cursor keys to navigate within pages"

Note that this is an accessibility feature of Firefox.

• http://kb.mozillazine.org/Scrolling_with_arrow_keys_no_longer_works
• http://kb.mozillazine.org/Accessibility_features_of_Firefox