@benofbrown that's a job for userchrome.css

google how to create it; basically make a chrome folder in your profile and a blank file called userchrome.css

then add this code to it:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* remove horizontal tab bar */
#TabsToolbar { visibility: collapse; !important; }

#tabbrowser-tabs { visibility: collapse !important; }

save and done.

the post at the start of this thread shows how my firefox looks after applying about 45 such userchrome tweaks. it's very powerful and the most importand way to customize the interface now that add-ons can't do much of that