I mostly use Google Chrome these days, but still have Mozilla FireFox installed as a browser, which used to be my standard browser before I switched to Chrome.
Today I launched FireFox again and was surprised to see something called Skipity in its toolbar. Furthermore, when I tried to go to my browser custom start page (a page with my most useful links) it took me to the Skipity website. A Google search showed that Skipity comes as part of an add-on called “Download Youtube video 12.0”. I removed that add-on, restarted FireFox, opened the URL I previously had as the browser start page and went to “Tools > Options > General > Startup” to select that URL as the start page again.
Any software that changes the start page of the browser without your consent should be permanently banned from your computer!