It is possible that the screen is too wide or too high and that the scroll bars fall off.
Open the system menu via Alt+Space and see if you can resize that window.
If that works then close Firefox (File > Exit/Quit) to save that setting.

See also:

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

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

• http://kb.mozillazine.org/localstore.rdf

I had this problem and it only started after installing firefox v12.0 (on win xp). Mozilla mentions it is related to the css 'overflow' attribute not being defined correctly.

HOWEVER, in my case, the offending line of css belonged to the jquery ui library file: jquery-ui-1.7.2.custom.css

.ui-helper-hidden-accessible { position: absolute; left: -99999999px; }

all I needed to do was remove one of the '9's and the horizontal scroll bar reappeared on all pages that included this jquery ui lib.

the better solution would be to upgrade to the latest jquery ui lib.

I give up ...

Id never thought I say this, but, Im advising my clients to change to IE. I feel like Ive joined forces with the devil. But I dont have any choice. FF has been a great disappointment...