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The idea is pretty simple. Highlight any word on a webpage and right-click it. Choose the Click SEAlang search option from the context menu. This pops open the Click SEAlang panel on the left side of the browser (or alternately on the bottom of the browser). If it's an English word you've selected, you can do a reverse-search in SEAlang's Thai, Khmer or Lao dictionaries. If you've selected a Thai, Khmer, or Lao word, it can tell by looking at the Unicode which language the word is and will look it up in the appropriate dictionary.
After you've installed the extension, the SEAlang bullseye logo sits in the bottom
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Be sure to set your preferred reverse-search language in the settings. You can find that under the Tools menu in Firefox, and then selecting Add-ons, then clicking the Options button for the Click SEAlang extension. If you're like me, even when you're reading in English, you'll run across a word and say to yourself, "I wonder what the Thai word for that is..."
The answer's a click away.
[More screenshots available here.]