does this happen to all windows or just the browser? If it's just
the browser, then I have no idea, sorry - but if it's all windows, you
may just have lost the Window List from your panel. If so, right click
on an empty part of the panel and, from the pop-up menu, select "Add to
panel". In the window that appears, scroll down until you reach "Window
List". Click on this to highlight it and then click "Add" and close the
window. Now you should have a double vertical row of dots on the panel
where you clicked at the start - left click on these and select "Move"
to put the window list where you want it and all should now be good.