The Instantfox Add-On disappears regularly!

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http://askubuntu.com – Instantfox is an add-on which you can use to search serveral sites with the adress-bar. For example you type "w linux" and it searches for the linux article on wikipedia. This also works with youtube, gmaps and many other sites. I installed it once, it worked directly, i didn't have to restart firefox. And then, some days later I typed "w ..." pressed enter and there was a google search for "w ..." instead a wikipedia search for "...". So Instantfox didn't work, i thought. I looked in the add-ons menu, and it wasn't there anymore. I installed it again and some days later it disappeared again. (HowTos)