hello, this looks like an issue with adware/malware that is active on your pc. please run a full scan of your system with the security software already in place and different other tools like the free version of malwarebytes, adwcleaner, microsoft safety scanner & kaspersky security scan.

Troubleshoot Firefox issues caused by malware

Thanks for reply, I'll do full scan and see the result.

phillipp:

My guess is that it's a pop up of some kind. I'm been seeing this quite often lately when I browse on Aurora without Adblock Plus.

Madi888:

Can you install Adblock Plus from http://adblockplus.org? That's a GREAT pop up blocker that literally blocks any ad or pop up. I use it daily and I've blocked well over 2,000 ads. You should try philipp's suggestion as well as it could be adware/malware.