overlapping divs in mozilla

Browsers render flow differently when an element does not establish its own layout context. Since @rameshdaniweb has not posted markup, try these proven fixes and quick checks that go beyond the general hints from @MidiMagic and @zukilover.

Try one of these fixes on the parent container (choose one, not all):

/* modern: creates its own block formatting context */
.container { display: flow-root; }

/* classic clearfix */
.container::after {
  content: "";
  display: table;
  clear: both;
}

/* overflow hack (works but may add scrollbars) */
.container { overflow: auto; }

/* if children are positioned absolutely, anchor them */
.container { position: relative; }

If floats are used, the clearfix or flow-root will make the parent wrap its children; overflow:auto also forces a BFC but can clip content. If absolute positioning is in play, ensure the parent is the positioned ancestor so siblings do not overlap.

Troubleshooting steps: open Firefox Inspector, select the container and check computed height, float, position, and overflow; toggle each fix to see which removes the overlap. If those do not help, post a minimal reproducible example (only the HTML and the CSS that affects the two divs). Background reading: Block formatting context and overflow. For the common clearfix pattern see CSS-Tricks clearfix.