Absolutely essential. Along with TreeTabs and LeechBlock this is one of my most-used addons. Love it.

I can't find a way to contact the developer so I'll ask here: Is there any way to get this to work within pdfs in the browser? Additionally, do you know of any similar modification for standalone pdf readers? Thank you so much!

Edit: I would like to note that it is possible to convert a pdf to html and then browse it but this is less than ideal.

I was highly skeptical that you're DDoS-ing Wikipedia. Since you don't allow user to set triggering key, instead making requests every time selecting on every length of text. But after I saw the download/users amount, I dispelled my concerns.
To be fair, it is a little bit annoying.

This add-on works perfectly. I'm always impressed by its ability to find the Wikipedia article I'm looking for even when the selected text itself doesn't match the title of the article exactly (or, in some cases, even closely).

This is, without question, my favorite and most used Firefox add-on, and I can't imagine browsing the internet without it anymore.