Window sizes and positions are stored in localstore.rdf in the Profile Folder.

Delete localstore.rdf or rename the file to localstore.rdf.sav in the Profile Folder to test if the file is corrupted.

See http://kb.mozillazine.org/Corrupt_localstore.rdf
(caution: do not delete the localstore.rdf file in the Firefox program installation folder)

Thanks for the reply. I did as you suggested (ie delected the localstore.rdf file in the Profile folder) and restarted Firefox, but the vertical scrollbar is still missing.

You did find the correct location of the hidden profile folder ?
Help > Troubleshooting Information > Profile Directory > Open Containing folder

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

You can use "Disable all add-ons" on the Safe mode start window to disable all extensions.

You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Thanks Cor-el...I did as you suggested and it was the Similarweb add-on theat was causing the scroll bar to disappear. I uninstalled it and problem solved.

You're welcome

I have a Mac with Firefox 5.0.1. I had the same problem with the vertical scrollbar. After reading your post I began disabling my most recent add-ons. Findbar Tweak 0.5.2 was the problem. Thanks for the help!

Thanks, Fuzuku. I disabled FoxClocks 2.9.35 because it was my most recently updated add-on. Problem solved, vertical scrollbar has returned.