I have been wanting this feature for a long time. I keep too many tabs open and slows FF down. With this great item I can get a list of URL's and save them in a text file for future reference. With more an more censorship on searches re-locating a URL is becoming more difficult. This is Great Great Great.
I have been saving URL's to a text file one at a time, when I first used this tool, I had 528 tabs opened.
Have been asking FF to add this feature for some time.