rendering method firefox

i am building plans member javascript. twin layers rendered separately: forehead background.

to establish mandatory distance background:

1. render forehead


2. measure tallness result


3. render forehead the
   background together

in formula looks this:

var forehead = renderforegroundintostring();
parentdiv.innerhtml = foreground;
var tallness = parentdiv.children[0].clientheight;
var credentials = renderbackgroundintostring(height);
parentdiv.innerhtml = forehead + background;

using ie7, square cake. however, firefox2 unequivocally pacific news parentdiv.innerhtml right away, therefore i can't review out forehead height.

when does firefox govern digest i check credentials date cell forehead digest completed, there any choice proceed establish tallness forehead elements?

[appended after contrast dan's answer (thanx dan)]

within physique callback slight (called behind settimeout(...)) i see, digest innerhtml still complete.