When does this happen?

Type about:preferences#general<enter> in the address bar.

There are two settings;

• When Firefox Starts

When the browser is started, what do you want to display. Many of us choose Show My Home Page.

• Home Page

When a new page is opened, what do you want displayed.

1. Use Current Page. Use what ever page(s) are open at that time.
2. Use Bookmark
3. Restore To Default. about:home shows a Mozilla home page with tools.

You can use any of these that you wish;

about:home (Firefox default home page), about:newtab (shows the sites most visited), about:blank (a blank page),

or you can enter any web page or about: page you want.

Most likely this is a bad browser add-on, with possible external support malware that makes it hard to remove. Here's my suggested procedure for tracking down and cleaning up bad add-ons and other hijackers. I know it seems long, but it's really not that bad.

(For Windows Vista, 7, 8, 8.1, 10; XP is somewhat different)

(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. This can help in smoking out undisclosed bundle items that snuck in with some software you agreed to install. 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) 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. All extensions are optional; none come with Firefox.

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.

Any improvement?

(3) You can search for remaining issues 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.

If the malware changed any settings, such as your home page or default search engine, let us know and we can advise on how to finish the clean-up.