Work on fixing the two issues is ongoing. The first is a problem with a slow browser script, which is also the most time-consuming issue since the Mozilla team needs around a week of telemetry data to evaluate the fix. This is also the primary reason they've delayed Firefox 49 in the first place. The second problem relates to loading Giphy GIF images on Twitter, which open in a new blank page instead of the Giphy URL. This issue was first detected in Firefox 49 Beta releases.
Firefox 49 is an important release in Mozilla's grand scheme of things when it comes to Firefox. This is the version when Mozilla will finish multi-process support rollout (a.k.a. e10s, or Electrolysis), and the version when Firefox launches the new WebExtensions API that replaces the old Add-ons API, making Firefox compatible with Chromium extensions.