I use Redirector in Firefox to rewrite URLs to the old Wikipedia:

  {
      "description": "Old Wikipedia",
      "exampleUrl": "https://en.wikipedia.org/wiki/Main_Page",
      "exampleResult": "https://en.wikipedia.org/wiki/Main_Page?useskin=vector",
      "error": null,
      "includePattern": "^(https?://)([a-z0-9-]*\\.)wikipedia\\.org\\/wiki\\/([^?]+)(?!=\\?useskin=vector)",
      "excludePattern": "",
      "patternDesc": "Adds ?useskin=vector to any Wikipedia page",
      "redirectUrl": "https://$2wikipedia.org/wiki/$3?useskin=vector",
      "patternType": "R",
      "processMatches": "noProcessing",
      "disabled": false,
      "grouped": false,
      "appliesTo": [
          "main_frame"
      ]
  }