This is good! I started using this after an older Wiktionary plugin whose name I forgot stopped working when Firefox stopped supporting XUL. With that older extension, you could right-click a highlighted word and click a menu option that gave you an on-page window showing the full Wiktionary page for that word. This plugin has a few other ways to trigger looking up words - a neat optional button that appears over highlighted words, a toolbar button you can click to look up the highlighted word, or a hotkey (CTRL-Q). The page that pops up is a modified and shortened Wiktionary page. If the highlighted word is not on Wiktionary, the add-on can still often find the correct translation by looking it up in some other source.

For me, there are only two things that make this a 4/5 instead of a 5/5. First, the modified Wiktionary page that this plugin displays is missing a few things, such as translations. Second, and most importantly, the hotkey is not editable in any obvious way, and in Firefox on macOS it is set to CMD-Q, which is also the system hotkey to close programs on macOS. The extension overrides the system hotkey, which means that you can no longer quit Firefox in the most convenient way with the extension enabled. (Oh yeah, and the plugin defaults to the Vietnamese Wiktionary version, but this is very easy to change. :) )