Hi Mike,
When you include in an HTML block any object (usually a picture, but could also be a table, etc.) whose width exceeds the default width of the left or right blocks, FireFox and MSIE differ in their display behaviour.
Firefox will automatically extend the width of all the blocks in that column in order to accomodate the wider object existing in one of the blocks. MSIE will display the over-wide object in its HTML block but will not extend the width of the blocks, hence the unpleasant "pushover" resulting effect.
Reason: a difference in the interpretation of the CSS rules between those two browsers.
In the case a hand, it is the Match Up "game" which you have included in one HTML block on the right hand side of your site's front page which is the culprit. When you included it (from http://www.thefreedictionary.com/) you selected as width: 225px. But the default width of blocks in Moodle is min. 160px and max. 210px (for the front page).
Remedy: I have not found a way to do this by tweaking the CSS in your theme. However, there is a solution which is only valid for the front page of your site (not if you have the same problem inside courses themselves).
You'll have to hack file \moodle\index.php, right at the beginning of the file, locate:
/// Bounds for block widths on this page
define('BLOCK_L_MIN_WIDTH', 160);
define('BLOCK_L_MAX_WIDTH', 210);
define('BLOCK_R_MIN_WIDTH', 160);
define('BLOCK_R_MAX_WIDTH', 210);
Change e.g. define('BLOCK_R_MIN_WIDTH', 235);
and... your Match Up block now fits inside its HTML block and all the other blocks take up the same width (in MSIE). This hack does not change anything to the (desired) display in FireFox.
QED,
Joseph