The page has a scrollbar but you don't know which element on page creates that scrollbar.
The Firefox Developer Tools allows us to see all elements causing overflow.
In Firefox, open its Developer Tools
Under the Inspector tab, if an element has a scrollbar (by using either `overflow: scroll` or `overflow: auto` but the content is longer than the element dimension), it will be shown with a `scroll` badge.
In the following screenshot, the highlighted element is displayed with the `scroll` badge indicating that it produces the scrollbar. Clicking the `scroll` badge will reveal the associate element.