An adware has infected 2 browsers

1. Maybe try to remove your personal files (or backup them first if you need to). They are located in:

~/.mozilla/firefox
~/.config/google-chrome

and some caches:

~/.cache/mozilla/firefox
~/.cache/google-chrome

Both of these browsers will create new profiles for you at first run.

2. You can also examine system-wide addons, they are at:

/usr/share/mozilla
/usr/lib/mozilla  
/usr/lib/chromium-browser  

3. If its not the addons that couses problems, it also can be Man-in-the-middle attack - something is changing HTML/JavaScript on the arrival - check your proxy settings, your routing tables, your internet gateway... Check network requests in "Developer Tools" in those browsers (Firefox: Ctrl+Shift+Q, Chrome: Ctrl+Shift+I). Download some .html in terminal (wget <url>), check if its changed.

Thank all for responses.
I installed "clamav" and ran a full scan on my system; Unfortunately I forgot to tell it to save scan log, I thought it would do it itself; anyway, my problem solved and I don't know what was the real cause.

You should probably check your running processes. See how here.

Try Privacy Badger (this wont solve the problem but at least may stop your ads). Install clamav and rkhunter.

If I were you, I would, personally, reinstall the OS, there is allways a new ubuntu version comming up.