diff --git a/apps/wiki/views.py b/apps/wiki/views.py
index 5464ffe38..ebfe5b2de 100644
--- a/apps/wiki/views.py
+++ b/apps/wiki/views.py
@@ -37,10 +37,20 @@ from wiki.tasks import send_reviewed_notification, schedule_rebuild_kb
 log = logging.getLogger('k.wiki')
 
 
+def _split_browser_slug(slug):
+    """Given something like fx35, split it into an alphabetic prefix and a
+    suffix, returning a 2-tuple like ('fx', '35')."""
+    right = slug.lstrip(ascii_letters)
+    left_len = len(slug) - len(right)
+    return slug[:left_len], slug[left_len:]
+
+
 OS_ABBR_JSON = json.dumps(dict([(o.slug, True)
                                 for o in OPERATING_SYSTEMS]))
-BROWSER_ABBR_JSON = json.dumps(dict([(v.slug, v.show_in_ui)
-                                     for v in FIREFOX_VERSIONS]))
+BROWSER_ABBR_JSON = json.dumps(
+    dict([(v.slug, {'product': _split_browser_slug(v.slug)[0],
+                    'maxFloatVersion': v.max_version})
+          for v in FIREFOX_VERSIONS]))
 
 
 def _version_groups(versions):
@@ -49,16 +59,9 @@ def _version_groups(versions):
     See test_version_groups for an example.
 
     """
-    def split_slug(slug):
-        """Given something like fx35, split it into an alphabetic prefix and a
-        suffix, returning a 2-tuple like ('fx', '35')."""
-        right = slug.lstrip(ascii_letters)
-        left_len = len(slug) - len(right)
-        return slug[:left_len], slug[left_len:]
-
     slug_groups = {}
     for v in versions:
-        left, right = split_slug(v.slug)
+        left, right = _split_browser_slug(v.slug)
         slug_groups.setdefault(left, []).append((v.max_version, right))
     for g in slug_groups.itervalues():
         g.sort()
diff --git a/media/js/showfor.js b/media/js/showfor.js
index a83ff08f1..e6ea4e72d 100644
--- a/media/js/showfor.js
+++ b/media/js/showfor.js
@@ -65,13 +65,38 @@ var ShowFor = {
             isSetManually;
 
         OSES = $osMenu.data('oses');  // {'mac': true, 'win': true, ...}
-        BROWSERS = $browserMenu.data('browsers');  // {'fx4': true, ...}
+        BROWSERS = $browserMenu.data('browsers');  // {'fx4': {product: 'fx', maxFloatVersion: 4.9999}, ...}
         VERSIONS = $browserMenu.data('version-groups');  // {'fx': [[3.4999, '3'], [3.9999, '35']], 'm': [[1.0999, '1'], [1.9999, '11']]}
         MISSING_MSG = gettext('[missing header]');
 
         // Make the 'Table of Contents' header localizable.
         $('#toc > h2').text(gettext('Table of Contents'));
 
+        // Given a symbol like 'm4' or '=fx4', return something like
+        // {comparator: '>', product: 'm', version: 4.9999}. Even if there's no
+        // explicit comparator in the symbol, the comparator that's assumed
+        // will be made explicit in the returned object. If it's not a known
+        // product/version combo, return undefined.
+        function conditionFromSymbol(symbol) {
+            var slug, browser, comparator;
+
+            // TODO: Put crappy special cases here.
+
+            // Figure out comparator:
+            if (symbol.substring(0, 1) == '=') {
+                comparator = '=';
+                slug = symbol.substring(1);
+            } else {  // If no leading =, assume >=.
+                comparator = '>=';
+                slug = symbol;
+            }
+            
+            browser = BROWSERS[slug];
+            return {comparator: comparator,
+                    product: browser.product,
+                    version: browser.maxFloatVersion};
+        }
+
         function updateForsAndToc(calledOnLoad) {
             // Hide and show document sections accordingly:
             showAndHideFors($osMenu.val(), $browserMenu.val());
@@ -97,25 +122,64 @@ var ShowFor = {
         // Set the {for} nodes to the proper visibility for the given OS and
         // browser combination.
         //
-        // Hidden are {for}s that {list at least one OS but not the passed-in one}
-        // or that {list at least one browser but not the passed-in one}. Also, the
-        // entire condition can be inverted by prefixing it with "not ", as in {for
-        // not mac,linux}.
+        // Hidden are {for}s that {list at least one OS but not the passed-in
+        // one} or that {list at least one browser expression but none matching
+        // the passed-in one}. Also, the entire condition can be inverted by
+        // prefixing it with "not ", as in {for not mac,linux}.
+        //
+        // Takes a browser slug like "fx4" rather than a browser code and a
+        // raw floating-point version because it has to be able to take both
+        // detected browsers and slugs chosen explicitly from the <select>.
         function showAndHideFors(os, browser) {
             $container.find('.for').each(function(index) {
-                var osAttrs = {}, browserAttrs = {},
+                var osAttrs = {}, browserAttrs = {},  // TODO: Eliminate browserAttrs?
                     foundAnyOses = false, foundAnyBrowsers = false,
                     forData,
                     isInverted,
-                    shouldHide;
+                    shouldHide,
+                    browserConditions = [];
 
-                // Catch the "not" operator if it's there:
+                // Return whether the given browser slug matches any of the
+                // given conditions. Passing an unknown slug results in
+                // undefined behavior.
+                // TODO: Implement with a generic any() instead--maybe underscore's.
+                function meetsAnyOfConditions(slug, conditions) {
+                    // Return whether a slug (like 'fx4' or 'fx35') meets a condition like {comparator: '>' product: 'm', version: 4.9999}.
+                    function meets(slug, condition) {  // 'fx4' is matched by {comparator: '=', product: 'fx', version: 3.9999}?
+                        var browser = BROWSERS[slug];
+                        if (browser.product != condition.product) {
+                            return false;
+                        }
+                        switch (condition.comparator) {
+                            case '=':
+                                // =fx35 --> {comparator: '=' browser: 'fx', version: 3.9999}
+                                return browser.maxFloatVersion == condition.version;
+                            case '>=':
+                                // fx4 --> {comparator: '>=' browser: 'fx', version: 4.9999}
+                                return browser.maxFloatVersion >= condition.version;
+                            // Insert '<' here someday.
+                        }
+                        return false;
+                    }
+
+                    for (var i = 0; i < conditions.length; i++) {
+                        if (meets(slug, conditions[i])) {
+                            return true;
+                        }
+                    }
+                }
+                
+                function slugWithoutComparators(slug) {
+                    return (slug.substring(0, 1) == '=') ? slug.substring(1) : slug;
+                }
+
+                // If the data-for attribute is missing, return.
                 forData = $(this).data('for');
                 if (!forData) {
-                    // If the data-for attribute is missing, move on.
                     return;
                 }
 
+                // Catch the "not" operator if it's there:
                 isInverted = forData.substring(0, 4) == 'not ';
                 if (isInverted) {
                     forData = forData.substring(4);  // strip off "not "
@@ -126,15 +190,16 @@ var ShowFor = {
                     if (OSES[this] != undefined) {
                         osAttrs[this] = true;
                         foundAnyOses = true;
-                    } else if (BROWSERS[this] != undefined) {
+                    } else if (BROWSERS[slugWithoutComparators(this)] !== undefined) {
                         browserAttrs[this] = true;
+                        browserConditions.push(conditionFromSymbol(this));
                         foundAnyBrowsers = true;
                     }
                 });
 
                 shouldHide = ((foundAnyOses && osAttrs[os] == undefined) ||
-                             (foundAnyBrowsers && browserAttrs[browser] == undefined)) &&
-                             // Special cases ):
+                              (foundAnyBrowsers && !meetsAnyOfConditions(browser, browserConditions))) &&
+                             // Special cases:
                              // TODO: make this easier to maintain somehow?
                              // Show android/m4 on desktop selection
                              !(osAttrs['android'] && os !== 'maemo' /* only one mobile browser ATM */) &&
@@ -143,10 +208,9 @@ var ShowFor = {
                              !(osAttrs['win'] && (os === 'android' || os == 'maemo') && (browserAttrs['fx4'] || !foundAnyBrowsers)) &&
                              !(browserAttrs['fx4'] && browser === 'm4' && (osAttrs['win'] || !foundAnyOses));
 
-                if ((shouldHide && !isInverted) || (!shouldHide && isInverted)) {
+                if (shouldHide != isInverted) {
                     $(this).hide();  // saves original visibility, which is nice but not necessary
-                }
-                else {
+                } else {
                     $(this).show();  // restores original visibility
                 }
             });