PriceGong is adware and it is not really extension -based in all cases. Check your Control panel Add/Remove programs first. You might have to resort to anti-malware. Best to my knowledge, Adwcleaner and Spyhunter detects and removes PriceGong. In some cases you can delete PriceGong files manually or with RevoUninstaller.
The following is the standard advice relating to malware. The problem with adware and toolbars is that some people do deliberately install such stuff and it does not always trigger malware detection tools.
Do try all the tools, and ensure you try up-to-date versions
Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.
You can try these free programs to scan for malware, which work with your existing antivirus software: