From Jimbojw.com

// This is a Greasemonkey Userscript. You may choose to install it or download the raw source.

Important! Unless otherwise specified, this userscript is released under The MIT License. If you choose to install it, you do so at your own risk and discretion.

// MS Report Server Fixes
// version 0.1
// 2006-12-14
// Copyright (c) 2006, Jim R. Wilson (wilson.jim.r at gmail)
// Released under The MIT License (http://www.opensource.org/licenses/mit-license.php)
//
// Purpose:
//    Tweaks Microsoft Report Server pages to render properly in Firefox.
//
// ==UserScript==
// @name          MS Report Server Fixes
// @namespace     http://jimbojw.com/userscripts/
// @description   Tweaks Microsoft Report Server pages to render properly in Firefox.
// @include       https://*/Reports/Reserved.ReportViewerWebControl.axd?*
// @include       http://*/Reports/Reserved.ReportViewerWebControl.axd?*
// @include       https://*/Reports/Pages/Report.aspx?ItemPath=*
// @include       http://*/Reports/Pages/Report.aspx?ItemPath=*
// ==/UserScript==
 
// Anonymous function wrapper
(function() {
 
/**
 * When the window is finished loading, start tweaking.
 */
window.addEventListener('load', function(event) {
 
    if (document.getElementsByTagName('iframe').length) {
        // Add a new style node to make sure all iframes are affected - even those not yet in the DOM.
        var style = document.createElement('style');
        style.appendChild(document.createTextNode('iframe{position:absolute;z-index:1}'));
        document.getElementsByTagName('head')[0].appendChild(style);
    }
    if (document.getElementsByTagName('style').length) {
        // Grab the inline style element and filter out bad values.
        var styles = document.getElementsByTagName('style');
        for (var i=0; i<styles.length; i++) {
            var style = styles[i];
            style.innerHTML = style.innerHTML.replace(/overflow-x:hidden;?/,'');
        }
    }
 
}, 'false');
 
})(); // end anonymous function wrapper