problem w/new Firefox - tables on our website

Common culprits when a page that used to render correctly suddenly shows the main content pushed far down are: invalid table/HTML structure (unclosed tags), a large top-level element such as an ad iframe or spacer row with a fixed height, uncleared floats, or a DOCTYPE/quirks-mode change that makes Firefox follow standards more strictly. Notes from @frosty714 and @MidiMagic point toward a large element near the top (ads) and a very tall overall document — these are the places to inspect first.

Quick diagnostic steps:

1. Validate the markup with the W3C Markup Validation Service and fix any missing closing tags, mismatched table nests or stray TD/TR tags.
2. Use Firefox Page Inspector / DOM tools (Page Inspector docs) to hover elements and find any node whose computed height or margin/top offset is unexpectedly large. Look specifically for iframes, spacer TRs/TDs or inline height attributes.
3. Temporarily disable CSS (or toggle styles in the inspector) to see the raw structure. Grep the source for height= and iframe to spot hard-coded tall elements inserted by ad scripts.
4. Check for uncleared floats: an upstream float without a clearing wrapper can push following blocks down in some browsers.

Targeted fixes (apply after identifying cause):

/* clear floated parents */
.clearfix:after {
  content: "";
  display: table;
  clear: both;
}

/* constrain third-party ad slots */
.ad-slot {
  max-height: 250px;
  overflow: hidden;
}

/* ensure main area is not getting pushed by a stray margin */
#mainContent {
  clear: both;
  margin-top: 0;
}

If the problem remains, create a minimal test page (DOCTYPE + the small table/header area + CSS) and compare behavior in both Firefox and IE; stricter standards handling in newer Firefox releases commonly surfaces invalid HTML problems that older builds silently tolerated.