A couple of longstanding issues regarding CSS
float handling have been fixed with Firefox 42.
In the first bug, the
margin-bottom properties were wrongly applied when used in conjunction with the
float property under certain circumstances. In the second bug, a
<table> after a floated element was not cleared properly, resulting in overlapped content. In the third bug, two floated elements with different widths led to a misplacement of the subsequent element, resulting in overlapped content as well.
While those changes should align with the specification and other browsers’ behaviour, some pages may be broken if those are relying on the previous behaviour of Firefox. Actually, Mozilla’s own site is known to be affected.