Hm. Using Firefox 41, the 1st version allowed me to click links but gave no results. The updated version immediately loads an empty page and prints "true" :-/
ok, I found the issue. A new version will be up in about 15 minutes. Fixes the bookmarklet format for Firefox, and tweaks a few niggling issues in the Similarity script itself.
you're right. I don't know much about differences in the way Firefox handles bookmarklets in the new format. Even loading the revised script directly doesn't seem to work on FF anymore. I'll have to look into it.