Hi and welcome to the forum
It could be that you have some spyware toolbar or home page hijack virus loose on your PC. If not, and someone else has been using your account on your PC--this happens a lot, take a look at this page on the Firefox-Mozilla support page:
Startup, home page, tabs, and download settings | Firefox Help It will show you how to add/remove/modify one or multiple home pages on your browser's startup.
This should take care of it unless you have a virus; in which case you need to scan/remove any viruses found by your built-in Antivirus program (such as Norton, Avast, McAfee, TrendMicro, etc.). And it's a good idea to download the Free
MALWAREBYTES antispyware program from
Malwarebytes.org and scan specifically for spyware viruses. Many of those are known to scramble or hijack your home pages. And if you've installed any Toolbars in any of your browsers (IE11, Edge, Chrome, Firefox, Safari, Opera) with the exception of Google Toolbar, you are pretty much guaranteed to have from 1-1,000 spyware viruses in your computer. The symptom being home-page hijacking, which you can google and lookup or go to wikepedia.com and lookup.
That should take care of it and get you back on the road again!
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