Noticed this go by in my logcat output on my B2G trunk build (but I suspect it affects desktop builds as well): JavaScript Warning: "Found trailing token after pseudo-element, which must be the last part of a selector: '['. Ruleset ignored due to bad selector." {file: "resource://gre-resources/forms.css" line: 90} Looks like this is for line 90 layout/style/forms.css , shown here: > 89 input.uri-element-right-align:-moz-locale-dir(rtl) > .anonymous-div[dir=ltr], > 90 input.uri-element-right-align:-moz-locale-dir(rtl)::-moz-placeholder[dir=ltr] { > 91 text-align: left !important; > 92 } https://mxr.mozilla.org/mozilla-central/source/layout/style/forms.css Apparently the "::-moz-placeholder" has to go at the end there. This was added a few days ago in bug 737786.