There are thousands of Firefox Add-ons https://addons.mozilla.org/en-US/firefox/ Most of them are good, but bad ones exist.
Recently, my Firefox was hijacked, when I was using Google, the search results page was redirected to http://www.search.com/
I was right, the problem was gone, Google search results page won’t be redirected to http://www.search.com/ anymore.
Next, I enabled add-ons one by one, if the problem arose again after enabling add-on A, then A was the evil add-on.
Finally, I found the bad guy – https://addons.mozilla.org/En-us/firefox/addon/google-no-tracking-url/
Here is a video recorded to show what it does to Firefox – the evidence.
The add-on download link is
So, guys, be careful next time you want to try a preliminarily reviewed Firefox add-on, which means “not thoroughly tested” and “potentially harmful”. And when you got problem with Firefox, follow my steps mentioned above, firstly, restart with all addons disabled, then enable one by one to find the cause.