Ah, great! Rated 5 out of 5 stars
It used to be that I could search with Wikipedia(en) in the URL bar and with either Wikipedia(fr) or the Mozillazine Knowledge Base in the Search bar but now (without this add-on) it seems that changing the one changes the other too.
I hope that it will work with SeaMonkey 2.23, currently «Mozilla/5.0 (X11; Linux x86_64; rv:26.0) Gecko/20100101 Firefox/26.0 SeaMonkey/2.23a1» ID:20130829003001 c-c:2e8458f26a91 m-c:aebdc69b02e5 even though when I try to install it AMO warns me that «I need Firefox 10.0 or higher» (then installs it if I okay the warning).
Edit: To make it work, you have to enter your search keyword(s) in the URL bar _and hit Enter_. Using the up-down arrows to select the search widget for the URL bar, or (of course) searching with the search bar, will get you your current search engine as set in the Search bar, in SeaMonkey's "Edit → Preferences → Browser → Internet Search", and in the about:config pref “browser.search.defaultenginename”.