Bug/Issue Page CSS - Width of search bar

So the problem is, that the quick search bar in the top right of this forum has a too high width, causing a horizontal scrollbar to appear for like 1 or maximum 2px space.

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I am using FireFox 55.0.3 on a 1920x1080 monitor, this happens in windowed, fullsize window and fullscreen mode.
In the forum I am using the default "Chucklefish Forums (light)" theme.

If I'd have to guess, it's because the overwritten width of 100% does not react to to the applied padding, causing it to leave it's appropriate space in the page's width layout.
This could either be resolved by letting the width of the box be calculated, as long as the padding has a fixed value, or by setting the box-sizing parameter to border-box, which could cause modifying needs for the height of the box.

EDIT:
As of no reaction was shown by now I continued testing. Since it works in Chrome just fine, I found out that it is one of the Firefox specific styling parameters. More specific, one of the older "fixes" (I assume) for Firefox's CSS rendering behaviour, is overwriting the fixed width of the search bar input field to 100%, causing the problem. Since the stylesheet is pre-assembled serversided I can't track down from where exactly the mistake is coming, but it's within the delivery of xenforo's stylesheet set.