I run 3.6.10 and the horizontal scroll bar vanishes.
I tried opening the browser in the Safe Mode & restoring the default settings. The horizontal scroll bar reappears until I resize the window. It vanishes again and does not reappear.
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Try this:
enter about:config into the url bar, accept warning if you haven't already.
Enter browser.fullscreen.autohide into filter. If set to true, double click to change to false. This should restore most horizontal scrollbars, unless a particular site has been badly programmed, e.g. purposely programmed iframes with scrollbar = no coding.
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 to save that setting.
See also http://kb.mozillazine.org/Resizing_oversize_window
If that didn't help then see http://kb.mozillazine.org/Corrupt_localstore.rdf