If it is a Firefox add-on, you may be able to disable or remove it here:

new Firefox button > Add-ons > Extensions

or

classic Tools menu > Add-ons > Extensions

You can also try this:

You can open the about:config page via the location bar and do a search for ask.com via the Filter at the top of that page.

You can reset all ask.com related prefs via the right-click context menu to their default values.

I think the problem may be fixed. Ask.com did show up in install/uninstall in Control Panel, and also I think it showed up under Options/General and I was able to make it stop hijacking the search function. However, I believe I've done these actions before and the problem recurs - perhaps when the next version of Firefox is installed? If so, could the Mozilla coders make it a separate addition or ask during installation whether the user wants it or not? I don't want to go through this every time I get a Mozilla update. Thanks!

could the Mozilla coders make it a separate addition or ask during installation whether the user wants it or not?

Mozilla does not include Ask toolbars or other products in any official release of Firefox.

Pardon my ignorance, but how does it get there, then? is it Google I should be complaining to?

Ask products usually are bundled with other downloads or add-ons. Sometimes you will see a checkbox somewhere along the way where you can opt out of installing it. In other cases, you might not be given a choice.

I'm confident that Ask would not come with a Google, Yahoo, or Microsoft/Bing add-on because they are competing search engines. I think other users have said Ask was included with the Foxit PDF Reader. I'm sure there are others.

I always opt out of it whenever I see that checkbox (& I can't remember which software it's been coming with) so I think it's sneaking in anyway. Infuriating. But for the time being I seem to be ok. Thanks for your help & info.