Do you still see any part of the scroll bar (i.e. only the buttons are gone) or is the scroll bar completely gone?

I still have the scroll bar and the slider button . It's the little buttons at the top and bottom of the slider bar that are missing .

Do they appear in other programs? Firefox would use the native OS scroll bars. It is also possible that the buttons are there, but just not visible. You can check that by clicking that area. If no buttons are there then the action will be page up or page down.

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 once had problems with my scroll bars. One day, I had reset Firefox, and found the scroll bars were restored.

Also check out; NewScrollbars (aka NoiaScrollbars) {web link} The "NewScrollbars"-extension replaces the default scrollbars of Firefox and Thunderbird with colorized ones (partly) known from "Noia themes".