hello, this sounds like you did download firefox from a bogus site which bundles the browser with unwanted software. genuine firefox downloads will not come with any third-party software and this is certainly no partner of mozilla. please only download firefox from the official location at mozilla!

• go into the windows control panel/programs section, sort it by install-date and remove all entries that were installed around the time you added the presumably adware-bundled firefox. also have a look for other unwanted toolbars...
• run a full scan of your system with different security tools like the free version of malwarebytes & adwcleaner.
• go to getfirefox.com and download a fresh installer for firefox. after installation go to firefox > addons > extensions and see if there are any toolbars listed - if so, reset the browser.

Remove a toolbar that has taken over your Firefox search or home page Troubleshoot Firefox issues caused by malware

Where did you download Firefox from?

Firefox for Windows, Mac OSX and Linux from www.mozilla.org has never come with bundled software, toolbars, trojans, virus, malware, adware and such as it is only Firefox.

If you prefer the full setup instead of the online stub installer on Windows then see https://www.mozilla.org/en-US/firefox/all/

A possibility is that it was installed by another application you installed or updated recently and thought after installing Firefox that it must have came with Firefox. Some download sites like download dot com aka cnet is a known adware site that with their installer bundles stuff with otherwise what was clean applications if you got it their own homepages.

Only eight thread results mentioning RegClean in Questions section in some way, one in Thunderbird section. If Firefox for Windows from mozilla.org did come with it, there would be MUCH more mentions. https://support.mozilla.org/en-US/search?q=RegClean&num_voted=0&num_votes=&asked_by=&answered_by=&q_tags=&created=0&created_date=&updated=0&updated_date=&sortby=1&a=1&w=2

Whenever you download a program, always get the program from it's home web site or a site that is trusted. Never be afraid to ask questions about any program.

Are you sure that this isn't a pop-up that you get when visiting an infected website?