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 to be 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 webpage or about: page you want.

In addition to the above :

OurLuckySites.com is a browser hijacker, malware that you'll need to
get rid of.

Go to the Firefox menu (≡) => Add-ons => Extensions

( Mac  : Command + Shift + A)

and look for any other unfamiliar or suspicious looking extensions.

When you find any : remove them.

Do the same here : for Windows => Control Panel for Mac and Linux => System Preferences

However : most malware will be 'hiding' somewhere , so let's run malware scans (even though you already have run some) :

Further information can be found in this article : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. They all work differently - what one program doesn't pick up, the other might.

Please take a look at this article : https://malwaretips.com/blogs/remove-ourluckysites-com/