The: left:-1000px; style for #skipnav element causes FF to add a horizontal scroll to page, adding: 

#skipnav {
  left:0;
}

to the base-rtl.css file could solve the problem.

Comments

farshid’s picture

farshid commented

also it could be:

#skipnav { right:-1000px; left: auto; }

if there is something special about the negative left value.