One notable change in Firefox 49 is the
#RGBA colour value support, which is affecting a Microsoft application and probably others. You may want to scan your stylesheets to prevent text being displayed as transparent.
Firefox 50 Beta enables the insecure login form warning that can still be seen on various sites. It’s time to encrypt your entire site to protect visitors.
Don’t forget that Firefox 52, currently on the Nightly channel, will finally drop the plug-in support except Flash. If your site has any content relying on a plug-in such as Silverlight or Java, you should have a plan to replace it as soon as possible.