Hi donflipke, sorry there was an issue in the 2.4.0 release causing the blank screen, and the fix (2.4.1) got stuck in the Firefox Addons review queue for ~20 hours.

But good news, the update has now been approved! If your Firefox has not received it yet, go to your Addon settings and select "Check for Updates" under the cog menu. After that everything should be working again!