This article intends to help theme authors update Firefox-3.5-compatible themes for Firefox 3.6 compatibility.
Chrome registration change
contents.rdf is not supported anymore, you need to use chrome.manifest instead.
Emptytext styling
XUL textboxes don't have the empty attribute anymore, but isempty instead. So instead of textbox[empty="true"], you need to use textbox[isempty="true"].
Right-to-left UI support
The [chromedir="rtl"] and [chromedir="ltr"] selectors have been obsoleted and won't work anymore on most elements. Instead, you need to use the new :-moz-locale-dir(rtl) and :-moz-locale-dir(ltr) selectors. See also: Making sure your theme works with RTL locales.
Cross-platform tabbed browser styling
The tabbed browser implementation no longer has a Mac-specific tabs-closebutton-box; instead, all platforms use the same names to identify the components of the tab strip.
Full Screen toolbar button
There's a new Full Screen toolbar button available from the Customize Toolbar dialog.