From MozillaZine Knowledge Base

If after an update your scrollbars disappear, there a few things you can try

• Update your theme or switch to the default one (Tools -> Themes).
• Disable your add-ons one at a time to see if they are the problem.[1]
• If you use Mac OS X, turn on the status bar (View -> Status Bar)[2]

The following won't help solve missing scrollbars but will help to navigate when missing or inaccessible

• Grab and Drag extension