diff --git a/toolkit/themes/linux/global/global.css b/toolkit/themes/linux/global/global.css
index 9328f7a66b..947b45107f 100644
--- a/toolkit/themes/linux/global/global.css
+++ b/toolkit/themes/linux/global/global.css
@@ -244,6 +244,9 @@ label[disabled="true"] {
 
 .tooltip-label {
   margin: 0;
+  word-wrap: break-word;
+  /* We must specify a min-width, otherwise word-wrap:break-word doesn't work. Bug 630864. */
+  min-width: 1px;
 }
 
 .header {
diff --git a/toolkit/themes/windows/global/global.css b/toolkit/themes/windows/global/global.css
index 573245296e..0712d19a4f 100644
--- a/toolkit/themes/windows/global/global.css
+++ b/toolkit/themes/windows/global/global.css
@@ -254,6 +254,9 @@ label[disabled="true"]:-moz-system-metric(windows-classic) {
 
 .tooltip-label {
   margin: 0;
+  word-wrap: break-word;
+  /* We must specify a min-width, otherwise word-wrap:break-word doesn't work. Bug 630864. */
+  min-width: 1px;
 }
 
 .header {