You can remove SafePrice Extension completely from Firefox browser. Till 2016 version, Avast has offered SafePrice to users as an option in Online Security add-on, which no longer the case from Avast 2016 release onwards, it is a standalone or separate add-on.
For instance, after installing the latest Avast Nitro update, you’ll be asked to install it when you open Firefox and Chrome browsers (if you don’t allow, the extension will be disabled), if you’ve installed this adware that Avast includes in its antivirus in a hurry, here is how you can remove it.
Uninstall or remove Avast SafePrice extension
I. For Chrome, click menu, select More Tools > Extensions and uninstall SafePrice extension.
II. 1. In Firefox, you can only disable the extension, but you can’t remove it. To deactivate it, you need to visit the add-ons manager.
2. After that, to remove SafePrice completely, close Firefox browser, navigate to this location on your computer: C:\Program Files\AVAST Software\Avast\SafePrice and delete FF folder and its contents.
Hope that helps.