If scroll bars disappear when you make the width smaller (i.e. too small) then this is usually cause

If scroll bars disappear when you make the width smaller (i.e. too small) then this is usually caused by one of the toolbars that have content that should be visible. Some extensions can place extra content on toolbars and Firefox may otherwise think that the width is too small. You can test that by hiding toolbars of possibly removing items placed on toolbars.

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