Most of the headlines announce the availability of a built-in PDF viewer, which increases its functionality and also contributes to making the browser more secure.
However, there is plenty of progress from a developer’s perspective, as well. Thus, the debugger has been improved and now supports pausing on exceptions and hiding non-enumerable properties.
The Remote Web Console is now available for connecting to Firefox on Android or Firefox OS. This tool is disabled by default, though, because it is in experimental stage; it can be turned on from about:config by setting the “devtools.debugger.remote-enabled” preference to true.
Starting this revision the Web Console CSS links are now opened in the Style Editor.
Further development available in Firefox 19 includes support for CSS @page, which is used to modify CSS properties (such as margins, orphans, widows and page breaks) when printing a document. Full release notes are available here.