Firefox 10 decided to auto hide the Forward button when there's no forward.
Very annoying. Because, as you navigate among the pages, the movement of these icons is distracting.
there doesn't seem to be a built-in way to disable this auto-hide. But, one work around is to set the icons to be small. Right click on a empty area in the tool bar, then choose 〖Customize…〗. Then check mark 〖Use Small Icons〗.
Reference: Source Firefox 10 release notes.