This week's Firefox 53.0 beta includes some theme changes so that lightweight themes are applied to private browsing windows and there are two new "compact" themes in Firefox (dark and light). Firefox's Reader Mode now also displays estimated reading times for a web page.
On the Windows side there are a lot of changes, with macOS they now have switched to being 64-bit only, and on the Linux side they have dropped support for processors older than the Intel Pentium 4 or AMD Opteron.
One other cool change with the Firefox 53.0 beta is support for WebM videos with alpha to play videos with transparent backgrounds.
More details on the Firefox 53.0 beta via Mozilla.org.