diff --git a/browser/base/content/test/browser_bug517902.js b/browser/base/content/test/browser_bug517902.js
index 02338f31c40..3a9af404959 100644
--- a/browser/base/content/test/browser_bug517902.js
+++ b/browser/base/content/test/browser_bug517902.js
@@ -31,7 +31,7 @@ function test() {
 
   content.location =
     "data:text/html," +
-    "<style type='text/css'>#test-image,#not-test-image {background-image: url('about:logo?c');}</style>" +
+    "<style type='text/css'>%23test-image,%23not-test-image {background-image: url('about:logo?c');}</style>" +
     "<img src='about:logo?b' height=300 width=350 alt=2 id='not-test-image'>" +
     "<img src='about:logo?b' height=300 width=350 alt=2>" +
     "<img src='about:logo?a' height=200 width=250>" +
diff --git a/browser/base/content/test/tabview/browser_tabview_bug594958.js b/browser/base/content/test/tabview/browser_tabview_bug594958.js
index f570c251be6..0ac228a415b 100644
--- a/browser/base/content/test/tabview/browser_tabview_bug594958.js
+++ b/browser/base/content/test/tabview/browser_tabview_bug594958.js
@@ -15,12 +15,12 @@ function test() {
   }
 
   // very long page that produces black bars at the right
-  let html1 = '<html><body style="background-color: #00f;">' +
-              '<div style="background: #fff; width: 95%; height: 10000px; ' +
+  let html1 = '<html><body style="background-color: %2300f;">' +
+              '<div style="background: %23fff; width: 95%; height: 10000px; ' +
               ' margin: 0 auto;"></div></body></html>';
 
   // very short page that produces black bars at the bottom
-  let html2 = '<html><body style="background-color: #00f;"></body></html>';
+  let html2 = '<html><body style="background-color: %2300f;"></body></html>';
 
   let tests = [{
     url: 'data:text/html,' + html1,
diff --git a/dom/tests/mochitest/general/test_focusrings.xul b/dom/tests/mochitest/general/test_focusrings.xul
index d7a96188097..a61ff250982 100644
--- a/dom/tests/mochitest/general/test_focusrings.xul
+++ b/dom/tests/mochitest/general/test_focusrings.xul
@@ -189,7 +189,7 @@ SimpleTest.waitForFocus(runTest);
 <button id="b2" label="Button"/>
 <button id="b3" label="Button"/>
 
-<iframe id="child" src="data:text/html,&lt;html&gt;&lt;style&gt;* { outline: none; -moz-appearance: none; } #elem:focus { outline: 2px solid red; } #elem:-moz-focusring { outline: 1px solid blue; }&lt;/style&gt;&lt;div id='container'&gt;&lt;/html&gt;"/>
+<iframe id="child" src="data:text/html,&lt;html&gt;&lt;style&gt;* { outline: none; -moz-appearance: none; } %23elem:focus { outline: 2px solid red; } %23elem:-moz-focusring { outline: 1px solid blue; }&lt;/style&gt;&lt;div id='container'&gt;&lt;/html&gt;"/>
 
 <body xmlns="http://www.w3.org/1999/xhtml" style="height: 300px; overflow: auto;"/>
 
diff --git a/intl/uconv/tests/unit/test_bug90411.js b/intl/uconv/tests/unit/test_bug90411.js
index fa78df34287..abecd1bfe98 100644
--- a/intl/uconv/tests/unit/test_bug90411.js
+++ b/intl/uconv/tests/unit/test_bug90411.js
@@ -7,7 +7,7 @@
  * 我听见 我忘记; 我看见 我记住; 我做 我了解。
  */
 
-const sample = "~{NJK{K-JGI59OAK~}5~{7VVS!#5+JG#,K{K-2;R*GsHTH;JGR;8vI59OS@T6!#~} ~{NRL}<{~} ~{NRM|<G~}; ~{NR?4<{~} ~{NR<GW!~}; ~{NRWv~} ~{NRAK=b!#~}";
+const sample = "~{NJK{K-JGI59OAK~}5~{7VVS!%235+JG%23,K{K-2;R*GsHTH;JGR;8vI59OS@T6!%23~} ~{NRL}<{~} ~{NRM|<G~}; ~{NR?4<{~} ~{NR<GW!~}; ~{NRWv~} ~{NRAK=b!%23~}";
 
 const expected = "\u95EE\u4ED6\u8C01\u662F\u50BB\u74DC\u4E865\u5206\u949F\u3002\u4F46\u662F\uFF0C\u4ED6\u8C01\u4E0D\u8981\u6C42\u4ECD\u7136\u662F\u4E00\u4E2A\u50BB\u74DC\u6C38\u8FDC\u3002 \u6211\u542C\u89C1 \u6211\u5FD8\u8BB0; \u6211\u770B\u89C1 \u6211\u8BB0\u4F4F; \u6211\u505A \u6211\u4E86\u89E3\u3002"; 
 
diff --git a/layout/style/test/chrome/test_moz_document_rules.html b/layout/style/test/chrome/test_moz_document_rules.html
index d53fb29b4a9..3c7ee51c13d 100644
--- a/layout/style/test/chrome/test_moz_document_rules.html
+++ b/layout/style/test/chrome/test_moz_document_rules.html
@@ -46,9 +46,10 @@ function run()
     function test_document_rule(urltests, shouldapply)
     {
         var zIndex = ++zIndexCounter;
-        var rule = "@-moz-document " + urltests +
-                   " { #display { z-index: " + zIndex + " } }";
-        var sheeturi = "data:text/css," + encodeURI(rule);
+        var encodedRule = encodeURI("@-moz-document " + urltests + " { ") +
+                          "%23" + // encoded hash character for "#display"
+                          encodeURI("display { z-index: " + zIndex + " } }");
+        var sheeturi = "data:text/css," + encodedRule;
         set_user_sheet(sheeturi);
         if (shouldapply) {
             is(cs.zIndex, zIndex,
diff --git a/layout/style/test/test_parse_rule.html b/layout/style/test/test_parse_rule.html
index 338d6541924..4b95f258b9e 100644
--- a/layout/style/test/test_parse_rule.html
+++ b/layout/style/test/test_parse_rule.html
@@ -104,13 +104,13 @@ base + "#6ident, #a {color: red }",
 ".\\--ident, .a { color: green; }",
 
 // CSS2.1 section 4.1.5 and 4.2
-"@import 'data:text/css,#a{color:green}';",
-"@import \"data:text/css,#a{color:green}\";",
-"@import url(data:text/css,#a{color:green});",
-"@import 'data:text/css,#a{color:green}' screen;",
-base + "@import 'data:text/css,#a{color:red}' blahblahblah;",
-"@import 'data:text/css,#a{color:green}'",
-"@import 'data:text/css,#a{color:green}",
+"@import 'data:text/css,%23a{color:green}';",
+"@import \"data:text/css,%23a{color:green}\";",
+"@import url(data:text/css,%23a{color:green});",
+"@import 'data:text/css,%23a{color:green}' screen;",
+base + "@import 'data:text/css,%23a{color:red}' blahblahblah;",
+"@import 'data:text/css,%23a{color:green}'",
+"@import 'data:text/css,%23a{color:green}",
 "@foo {}" + base,
 "@foo bar {}" + base,
 "@foo; " + base,
diff --git a/layout/tools/reftest/reftest.xul b/layout/tools/reftest/reftest.xul
index dec48704b78..58201761811 100644
--- a/layout/tools/reftest/reftest.xul
+++ b/layout/tools/reftest/reftest.xul
@@ -36,7 +36,7 @@
    - ***** END LICENSE BLOCK ***** -->
 <?xml-stylesheet type="text/css" href="data:text/css,
 
-#_box_windowsDefaultTheme:-moz-system-metric(windows-default-theme) {
+%23_box_windowsDefaultTheme:-moz-system-metric(windows-default-theme) {
   display: none;
 }
 
