You should be able to disable an unwanted extension on the Add-ons page. Either:

• Ctrl+Shift+a
• orange Firefox button (or Tools menu) > Add-ons

In the left column, click Extensions. Then look for the disable button. There may also be a remove button, it depends on how it was installed. If there's no remove button, check the Windows Control Panel.

Better Surf seems to arrive on a computer with other software. You should take a hard look here:

Windows Control Panel > Add/Remove Programs

Click the install date column heading so you can see what was installed together. Some of these are obviously not what you wanted, but the problem is, some free software is hardwired to keep putting the junk back on your computer. So consider removing EVERYTHING that was bundled together.

This support article recommends malware cleaners to further scrub your system: Troubleshoot Firefox issues caused by malware.

Hopefully with these actions you will get rid of it in one try rather than having to fight it for days and days.

Note that the recent version of BetterSurf malware has just been added to blocklist. https://addons.mozilla.org/en-US/firefox/blocked/