From what I've see here it has always been a matter of the window being too large or shifted to far to the right. In other one does not see four rounded corners of the window.

• see http://kb.mozillazine.org/Resizing_oversize_window

Sooner or later someone is going to have a problem where the scroll bars are actually missing regardless of window size and for that there is Grab and Drag (187.4KB). Enables grab and drag scrolling.  (Scroll without scrollbars)  [Tools for options][icon (toggle)].  Use the startup wizard in options to produce the toolbar toggle button.
2.8.0 add let/right flicks for back/forward, support for HTML5 contenteditable.</td>

This issue can be caused by the Babylon Toolbar extension

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes