Bravo, exactly what I needed. Removed 724 duplicate bookmarks and took a few mere minutes. Automatically pick oldest added duplicate worked for me.

After a Firefox account sync on a new machine, I was left with many duplicate bookmarks. This extension removed them, and then the empty folders they had been in, with just a few clicks. It took just a few moments. This was a great time saver.

Quickly handled 12,000 duplicate bookmarks caused by bugs in Xmarks

Highly recommended!

Probably the best extension to remove duplicates at present.

I had over 1322 duplicate bookmarks because of a syncornisation error and this simple addon did the job in less than a minute.
Simple design, easy to use and a top recommendation !

really helpful

Awesome and fast UI for selecting duplicates in one go, and just one click to delete... Really appreciate that only bookmark editing permissions were required! Suggested improvement: deletion process is slow with lots of disk usage (read/writes). Perhaps add option to process the deletion entirely in RAM?

Very simple, easy and straight to the point!