Moz-extension entry in Firefox history

Recently I noticed an odd thing appearing in my Firefox history. Whenever I would first start the browser, a line would appear in history:

moz-extension://25baf6db-c60b-4714-90e8-c029954fa2b6/_blank.html

A search yielded no result. I did find one discussion about a similar case where an add-on (extension) was causing this. I did other searches based on just “moz-extension” and “blank.html”. Eventually, I disabled a handful of add-ons and restarted Firefox. No odd entry! One by one I enabled them until I found the one that causes this entry:

“FBPurity”

I am writing to them to see if they know why this is happening.