The most important thing is, as you should be aware by now, the distrust of Symantec certificates including ones issued under the GeoTrust, RapidSSL, Thawte and Verisign brands. This measure affects various sites, so webmasters are highly encouraged to use Firefox Nightly to double check if your site won’t get a security error page.
By the way, this Saturday marks the 3rd anniversary of FxSiteCompat.com, and another milestone, 1,000 followers on Twitter, is also within reach. While scaling this kind of initiative is difficult, we have never wanted to remain as a mere documentation project. Join us if you’re interested in outreach, analysis or engineering!
Update: The distrust has been postponed until Firefox 64 Beta.