If you download Firefox from the official Mozilla download page, you won't get ask. Please go to www.getfirefox.com, and download Firefox.

Also, since you'd possibly been downloading a version of Firefox that has malware bundled in it from a malicious site, please Troubleshoot Firefox issues caused by malware can help.

You can open the about:config page via the location/address bar and search for prefs that refer to ask.com. You can reset user set (bold) prefs via the right-click context menu to the default value.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

Also possible that you installed or updated something recently that inserted ask.com into Firefox.

Firefox from mozillla.org does not come with ask.com

Also check you Homepage in Options as Firefox is a web browser and not a search engine like Google, Yahoo, Bing etc sites.