This article describes why you may be missing the vertical scroll bar on the right on the Firefox window.
No scroll bar on any page
Some themes can cause the scroll bar to disappear. In this case, switch back to the default theme.
At the top of the Firefox windowOn the menu bar, click on the Tools menu and select Add-ons. The Add-ons window will open.
At the top of the Add-ons window, select the Themes panel.
Click on Default and click Use Theme.
If Use Theme is disabled, you are using the default theme, so this isn't your problem.
Click on the Restart Firefox button that appears.
With Firefox back to using the default theme, the scroll bar should be back.
Down arrow missing on scroll bar
If the up arrow on the scroll bar is visible, but the down arrow is not, you may need to display the Status Bar. In the View menu, click to put a check mark by Status Bar. The down arrow should reappear.
No scroll bar on certain sites
If the error only occurs on certain sites, clear your cache and try loading that website again. If the problem remains, it may be a bug with the website (this is often because the page is wrapped in a block that has an inappropriate "overflow" style applied). You can report a broken website by opening the Help menu and choosing Report Broken Web Site....