Some time ago, I wrote a bookmarklet to automatically open a new tab for the English-language Wiktionary entry for a word, either when that word is selected or when I am on one of my more common language-specific dictionary sites. I have now used this bookmarklet some thousands of times, so I can say that it works pretty well (unless the website I'm on is doing some hinky thing with its text).

Wiktionary is a communally edited dictionary of all words in all languages, although the coverage of some languages is much better than others. The "English-language Wiktionary," thus, is the dictionary of all words in all languages defined in English.

To use the bookmarklet as-is, just drag this link to the bookmarks bar of your browser: find on Wiktionary

Here is the raw code:
javascript:var%20querystring="";%20var%20url=""+window.location;%20var%20querystring="";%20if(url.search("q=")>0){var%20querystring=url.split("q=")[1]};%20if(url.search("query=")>0){var%20querystring=url.split("query=")[1]};%20if(url.search("search=")>0){var%20querystring=url.split("search=")[1]};%20if(url.search("duden.de")>0){var%20pieces=url.split("/");%20var%20querystring=pieces[pieces.length-1];};%20if(url.search("woord=")>0){var%20querystring=url.split("woord=")[1]};%20if(url.search("engelsk/=")>0){var%20querystring=url.split("engelsk/")[1]};%20if(url.search("bab.la")>0){var%20pieces=url.split("/");%20var%20querystring=pieces[pieces.length-1].replace("-","%20");};%20var%20querystring=querystring.split("&")[0];%20var%20querystring=querystring.replace("+","%20");if(querystring==""){var%20querystring=window.getSelection().toString()};%20window.open("http://en.wiktionary.org/wiki/"+querystring,"_blank");%20void(0);

I have used this extensively in Firefox, and have also tested it in Chrome.