The problem might be with some extensions or something might have changed the search engine Read[[ https://support.mozilla.org/en-US/kb/use-search-bar-firefox-and-manage-search-engines |Manage search engines]] on how to remove and set search engines. If you find anything like porn or vulgar search engines in the above settings menu.Just remove it or change the search engine back to google. Hope it helps.

It sounds like this goes beyond just switching your search page. You should consider a full-scale malware cleaning.

(1) Open the Windows Control Panel, Uninstall a Program. After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, by date. Be suspicious of everything you do not recognize/remember, as malware often uses important or innocent sounding names to discourage you from removing it. Take out as much trash as possible here.

(2) You can supplement your regular security software with the scanning/cleaning tools listed in our support article: Troubleshoot Firefox issues caused by malware. These on-demand scanners are free and take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.

(3) Open Firefox's Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons
• in the Windows "Run" dialog, type or paste
  firefox.exe "about:addons"

In the left column, click Plugins. Set nonessential and unrecognized plugins to "Never Activate".

In the left column, click Extensions. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

(4) To restore your regular search engine, if necessary, see: Use the Search bar in Firefox and manage search engines.

Success?

There are a number of add-ons and third party programs that will block adult sites.

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for what you want.

Adblock Plus {web link} Blocks annoying video ads on YouTube, Facebook ads, banners and much more. Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).

Adblock Plus Pop-up Addon {web link} Adblock Plus Pop-up Addon extends the blocking functionality of Adblock Plus to those annoying pop-up windows that open on mouse clicks and other user actions.

Forum; Adblock Plus Homepage {web link}