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

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I am not going to do that. I will just continue to run 31.0. When I upgraded to 32.0, that is when the vertical scroll bar started to disappear.

Some info: When the scrollbar was gone, I could momentarily get it back if I resized the window, vertically. I do have the walnut app running.

That is possibly this bug:

• bug 1066934 - FF32: Skinned scrollbars thumb disappear upon scrolling a page at specific zoom percentage (different between FF themes)

Please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html