The change gerrit:292571, which introduced a "float: left" in order to fix T136945, has the potentially unwanted side-effect that the search input field has become a lot smaller in Firefox (but not in Chrome or IE/Edge).
Steps to reproduce:
(1) Open Special:Search in Chrome or IE/Edge and observe the "large" input field.
(2) Open Special:Search in Firefox and observe the "small" input field.
(3) Use developer tools to revert gerrit:292571 in Firefox and observe that the input field becomes "large" again.