Posting again because the problem persists but now I know it's not due to addons or extensions. It will only show messages on safe mode, although the left bar shows up fine. It's not necessary to check disable addons. The error console shows a number of messages like this one: Timestamp: 8/27/2015 8:58:48 AM Warning: Unknown property 'padding-start'. Declaration dropped. Source File: https://mozorg.cdn.mozilla.net/media/css/thunderbird-start-bundle.f3f2a61e7492.css Line: 1, Column: 2947 Source Code: /*! updated 2015-06-17 */@font-face{font-family:'Open Sans Light';src:url("/media/fonts/OpenSans-Light-webfont.804037562eab.eot?#iefix") format('embedded-opentype'),url("/media/fonts/OpenSans-Light-webfont.1c8075cacedb.woff") format('woff'),url("/media/fonts/OpenSans-Light-webfont.ecb4572a5e47.ttf") format('truetype');font-weight:normal;font-style:normal}@font-face{font-family:'Open Sans Light';src:url("/media/fonts/OpenSans-Semibold-webfont.0ea045029306.eot?#iefix") format('embedded-opentype'),url("/media/fonts/OpenSans-Semibold-webfont.26755c681f8f.woff") format('woff'),url("/media/fonts/OpenSans-Semibold-webfont.b5a0ae86dca8.ttf") format('truetype');font-weight:bold;font-style:normal}@font-face{font-family:'Open Sans Light';src:url("/media/fonts/OpenSans-LightItalic-webfont.6725fc490942.eot?#iefix") format('embedded-opentype'),url("/media/fonts/OpenSans-LightItalic-webfont.3d46a6efa1ad.woff") format('woff'),url("/media/fonts/OpenSans-LightItalic-webfont.26f1e68dfbd8.ttf") format('truetype');font-weight:normal;font-style:italic}@font-face{font-family:'Open Sans';src:url("/media/fonts/OpenSans-Regular-webfont.83efe33660ab.eot?#iefix") format('embedded-opentype'),url("/media/fonts/OpenSans-Regular-webfont.2696e36f12c5.woff") format('woff'),url("/media/fonts/OpenSans-Regular-webfont.3cbf4d3ed22e.ttf") format('truetype');font-weight:normal;font-style:normal}@font-face{font-family:'Open Sans';src:url("/media/fonts/OpenSans-Bold-webfont.7ae9b8ba7886.eot?#iefix") format('embedded-opentype'),url("/media/fonts/OpenSans-Bold-webfont.b9dc2e7aefec.woff") format('woff'),url("/media/fonts/OpenSans-Bold-webfont.5a100916f94b.ttf") format('truetype');font-weight:bold;font-style:normal}@font-face{font-family:'Open Sans';src:url("/media/fonts/OpenSans-Italic-webfont.e487b7cb0725.eot?#iefix") format('embedded-opentype'),url("/media/fonts/OpenSans-Italic-webfont.fdc3518c128e.woff") format('woff'),url("/media/fonts/OpenSans-Italic-webfont.9b30f13428e1.ttf") format('truetype');font-weight:normal;font-style:italic}html{color:#333;font-size:14px;line-height:1.5;font-family:'Open Sans',X-LocaleSpecific,sans-serif}body{margin:1.5rem;background:url("/media/img/sandstone/bg-gradient-sky.7ea325995978.png") repeat-x,url("/media/img/sandstone/grain.855f29e0c686.png") repeat,#eee}a{color:#0096dd;text-decoration:none}a:hover,a:focus,a:active{color:#00539f;text-decoration:underline}h1,h2{margin:0;font-family:'Open Sans Light',X-LocaleSpecific-Light,'Open Sans',X-LocaleSpecific,sans-serif;font-weight:normal}main{position:relative;overflow:hidden}header,section,footer{box-sizing:padding-box;margin-top:1.5rem;padding:0 1.5rem}header h1,section h2.iconic{position:relative}header h1:before,section h2.iconic:before{display:block;position:absolute;top:0;background-image:url("/media/img/thunderbird/start/sprite.23eaf0044898.png");background-repeat:no-repeat;content:''}footer a{display:inline-block;-moz-padding-start:25px;padding-start:25px}footer a:before{-moz-margin-start:-25px;margin-start:-25px}footer a:before,section a.iconic:before{display:inline-block;-moz-margin-end:5px;margin-end:5px;width:20px;height:20px;background-image:url("/media/img/thunderbird/start/sprite.23eaf0044898.png");background-repeat:no-repeat;vertical-align:text-bottom;content:''}header h1{box-sizing:padding-box;font-size:1.25rem;line-height:1}header h1:before{height:100px}header h1 span{display:block;color:#2666a6;font-size:3rem;letter-spacing:-0.1ex}section h2{font-size:1.25rem;line-height:1.3}section h2.iconic{display:table-cell;-moz-padding-start:55px;padding-start:55px;height:50px;vertical-align:middle}section h2:before{width:50px;height:50px}section.tip h2:before{background-position:0 -100px}section.addons h2:before{background-position:-50px -100px}section.help h2:before{background-position:0 -150px}section.blog h2:before{background-position:-50px -150px}section p{margin:.7em 0 0}section a[href$="Contributing"]:before{background-position:-40px -200px}section a.more:after{content:"\00A0\00BB";white-space:nowrap}footer ul{overflow:hidden;margin:0;padding:0;list-style-type:none}footer li{box-sizing:padding-box;padding-bottom:.5rem}footer a[href*="features"]:before{background-position:0 -200px}footer a[href*="blog"]:before{background-position:-20px -200px}footer a[href*="Contributing"]:before{background-position:-40px -200px}footer a[href*="addons"]:before{background-position:-20px -220px}footer a[href*="support"]:before{background-position:-40px -220px}footer a[href*="how"]:before{background-position:0 -220px}[dir="ltr"] header h1:before,[dir="ltr"] section h2.iconic:before{left:0}[dir="rtl"] header h1:before,[dir="rtl"] section h2.iconic:before{right:0}@media(min-width:1024px){footer li{width:50%;min-width:10em}[lang|="en"] footer li{width:33%}}@media(min-width:800px) and (max-width:1023px){footer li{width:50%}}@media(min-width:800px){main>header,main>div,main>footer{width:50%}[dir="ltr"] main>header,[dir="ltr"] main>div:last-of-type,[dir="ltr"] main>footer,[dir="ltr"] main>footer li{float:left}[dir="ltr"] main>div:first-of-type{float:right}[dir="rtl"] main>header,[dir="rtl"] main>div:last-of-type,[dir="rtl"] main>footer,[dir="rtl"] main>footer li{float:right}[dir="rtl"] main>div:first-of-type{float:left}}@media(min-width:640px) and (max-width:799px){header,section{width:70%}footer{position:absolute;top:120px;width:30%}[dir="ltr"] footer{right:0}[dir="rtl"] footer{left:0}}@media(min-width:480px) and (max-width:639px){footer li{width:50%;min-width:10em}[lang|="en"] footer li{width:33%}[dir="ltr"] footer li{float:left}[dir="rtl"] footer li{float:right}}@media(min-width:480px){header h1{display:table-cell;-moz-padding-start:110px;padding-start:110px;height:100px;vertical-align:middle}header h1:before{width:100px}}@media(max-width:479px){header h1{padding-top:110px;text-align:center}header h1:before{width:100%;background-position:center top}}