The issue is caused by bug 1473927 which increased the specificity of the toolkit textbox[type=search] rules. Since no-one is maintaining the DevTools XUL panels (hence having this bug slip in 2 releases), I think it's just more simple to stop using [type=search], and remove the associated DevTools styling.
Bug 1518159 Comment 1 Edit History
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The issue is caused by bug 1473927 which increased the specificity of the toolkit textbox[type=search] rules. Since no-one is maintaining the DevTools XUL panels (hence having this bug slip in 3 releases), I think it's just more simple to stop using [type=search], and remove the associated DevTools styling.