The Newsfox addon was giving me problems in Linux, so I could only use it when in Windows. The dreaded enlarged bar at the bottom of the browser would show up after installing Newsfox (even in a new install of SeaMonkey with clean profile). Today I finally figured it out after realizing that it worked correctly as root (Newsfox has to be installed as root, so I tested SeaMonkey again as root after closing it). So that sounded like a permissions problem to me, and so it was. The permissions on /opt/seamonkey/lib/seamonkey-1.1.12/chrome/newsfox.jar was originally 400, which means no read privilege to group or others. Changing it to 444 or 644 works.
HTH someone with a similar issue.
EDIT: 444 means giving adding read privilege to group and others, while 644 also adds write privilege to root, which seems to be the default for other extensions, AFAICT.