Simply switch off, the rest of Firefox even runs better. I'm not satisfied the way Firefox "pushed" this add on. I can very well install add on's myself.

Firefox has nothing to do with Ask.com, you didn't get it with a Firefox program download (from an official mirror server) or a Firefox program update.

If you're sure that Ask didn't get into Firefox from another program you installed or from installing a Plugin for Firefox, I wonder if there is another user of that PC who installed something that you aren't aware of. Do you see anything new or unexpected in the Control Panel > Add or Remove Programs (if you are on a Windows OS)??

Quote: I installed some free software by Arcsoft

You can do a search via the Filter bar on the about:config page and reset bold (user set) preferences to the default value.
See http://kb.mozillazine.org/about%3Aconfig

See also http://kb.mozillazine.org/Preferences_not_saved