Two platform-specific features have been added: the Windows version follows display scaling settings to render text larger on high-resolution displays, and the Mac version shows download progress in the Dock icon.
Other changes include improved WebGL rendering performance and word-wrapping of plain-text files (hurrah!).
Of the remainder, six are described as high impact, three as moderate and one low.
Eight of those fixes are also relevant to Firefox 17 ESR - the extended support version of the browser that gets security patches but no new features. Firefox ESR is intended for environments that don't want the trouble of having to perform compatibility testing on each mainstream Firefox release.
The current version of Firefox ESR is 17.0.7.
Those eight fixes have also been applied in Thunderbird 17.0.7.
Firefox usage has slipped to just under 20%, according to StatCounter, behind Chrome (41%) and Internet Explorer (almost 28%).
On StatCounter's figures, Firefox use peaked in late 2009 and has since lost share to Chrome and (to a lesser extent) Safari.