Rated 3 out of 5 stars
With this addon enabled, my firefox starts 10-20 seconds longer. Totally not worth it.
P.S. I'm using only 1 theme - lightning storm one. Used shuffler to keep it's animation.
Interesting, if you take a look at its source-code you'll notice what this add-on does is quite trivial, it merely reads a single Firefox preference option (containing the installed themes) and shuffles the contents.
Hence, i'd say whatever is happening is isolated to your system in case you want to investigate further (nobody else reporting such an issue) -- Hm, or perhaps you have installed way too many themes?
 : https://github.com/diegocr/PersonasShuffler/blob/master/bootstrap.js