After leaving Firefox unattended for some time, I sometimes get a blank bar,
like the menu/tool bar but with a slightly lighter background colour. It has no
context menu and doesn't respond to drag/drop from the Customize Toolbar window.
The DOM inspector flags the offensive element as

main-window\mainPopupSet\
label="xxx" width="41" height="19" id="aHTMLTooltip"/>

xxx changing each time. The same element on a "normal" Firefox window is

id="aHTMLTooltip"/>

Removing the extra attributes from the DOM inspector does not make any
difference, but removing the whole element recovers normal appearance.

If I could find a reliable way of reproducing the problem I would try safe mode.
"Suspect" extensions are the (official) Google Toolbar and Toolbar Cleanup, but
I have about 30 others. Firefox 1.5 on Windows XP Pro.