diff --git a/parser/html/javasrc/TreeBuilder.java b/parser/html/javasrc/TreeBuilder.java
index 06039af91b5a..c9385b801668 100644
--- a/parser/html/javasrc/TreeBuilder.java
+++ b/parser/html/javasrc/TreeBuilder.java
@@ -1281,7 +1281,7 @@ public abstract class TreeBuilder<T> implements TokenHandler,
                                     if (start < i) {
                                         accumulateCharacters(buf, start, i
                                                 - start);
-                                        start = i;
+                                        // start index is adjusted below.
                                     }
                                     /*
                                      * Parse error.
@@ -1296,7 +1296,7 @@ public abstract class TreeBuilder<T> implements TokenHandler,
                                     if (start < i) {
                                         accumulateCharacters(buf, start, i
                                                 - start);
-                                        start = i;
+                                        // start index is adjusted below.
                                     }
                                     /*
                                      * Parse error.
@@ -1320,13 +1320,19 @@ public abstract class TreeBuilder<T> implements TokenHandler,
                                     i--;
                                     continue;
                                 case AFTER_AFTER_FRAMESET:
+                                    if (start < i) {
+                                        accumulateCharacters(buf, start, i
+                                                - start);
+                                        // start index is adjusted below.
+                                    }
+                                    /*
+                                     * Parse error.
+                                     */
                                     errNonSpaceInTrailer();
                                     /*
-                                     * Switch back to the main mode and
-                                     * reprocess the token.
+                                     * Ignore the token.
                                      */
-                                    mode = IN_FRAMESET;
-                                    i--;
+                                    start = i + 1;
                                     continue;
                             }
                     }
diff --git a/parser/html/nsHtml5TreeBuilder.cpp b/parser/html/nsHtml5TreeBuilder.cpp
index 573bd14091cd..390ed92e2370 100644
--- a/parser/html/nsHtml5TreeBuilder.cpp
+++ b/parser/html/nsHtml5TreeBuilder.cpp
@@ -395,7 +395,6 @@ nsHtml5TreeBuilder::characters(const char16_t* buf, int32_t start, int32_t lengt
               case NS_HTML5TREE_BUILDER_IN_FRAMESET: {
                 if (start < i) {
                   accumulateCharacters(buf, start, i - start);
-                  start = i;
                 }
                 errNonSpaceInFrameset();
                 start = i + 1;
@@ -404,7 +403,6 @@ nsHtml5TreeBuilder::characters(const char16_t* buf, int32_t start, int32_t lengt
               case NS_HTML5TREE_BUILDER_AFTER_FRAMESET: {
                 if (start < i) {
                   accumulateCharacters(buf, start, i - start);
-                  start = i;
                 }
                 errNonSpaceAfterFrameset();
                 start = i + 1;
@@ -417,9 +415,11 @@ nsHtml5TreeBuilder::characters(const char16_t* buf, int32_t start, int32_t lengt
                 continue;
               }
               case NS_HTML5TREE_BUILDER_AFTER_AFTER_FRAMESET: {
+                if (start < i) {
+                  accumulateCharacters(buf, start, i - start);
+                }
                 errNonSpaceInTrailer();
-                mode = NS_HTML5TREE_BUILDER_IN_FRAMESET;
-                i--;
+                start = i + 1;
                 continue;
               }
             }
diff --git a/parser/htmlparser/tests/mochitest/html5lib_tree_construction/tests26.dat b/parser/htmlparser/tests/mochitest/html5lib_tree_construction/tests26.dat
index 182af997e01d..867bbeecd50c 100644
--- a/parser/htmlparser/tests/mochitest/html5lib_tree_construction/tests26.dat
+++ b/parser/htmlparser/tests/mochitest/html5lib_tree_construction/tests26.dat
@@ -396,3 +396,16 @@ Line 1 Col 19 Expected closing tag. Unexpected end of file.
 | <html>
 |   <head>
 |   <frameset>
+
+#data
+<!DOCTYPE html><html><frameset></frameset></html>XXX</frameset>
+#errors
+52: Non-space character in page trailer.
+52: Non-space character in page trailer.
+52: Non-space character in page trailer.
+63: Stray end tag “frameset”.
+#document
+| <!DOCTYPE html>
+| <html>
+|   <head>
+|   <frameset>