jQuery's .hide().show() results in an inline display:block style in Firefox (in Chrome it is the correct display:inline). Looking at the source code, when the element is first hidden, Firefox gets the computed display style for the element, stores it as a data attribute and restores as an inline style when unhidden. Apparently, in this case "block" is stored instead of inline. Somehow related to hiding the element when it is not in the DOM (so jQuery cannot store data on it); switching the order of append and hide fixes it.

Change 115561 had a related patch set uploaded by Gergő Tisza:
Fix display issues with "view terms"

https://gerrit.wikimedia.org/r/115561

Change 115561 merged by jenkins-bot:
Fix display issues with "view terms"

https://gerrit.wikimedia.org/r/115561