If you receive TypeError: avg_DTFox.prefs has no properties when Firefox starts after upgrading to Firefox 188.8.131.52, read these instructions on AVG's website.
The template "suggestFx3" does not exist or has no approved revision.
This issue is fixed in the new AVG program version 8.0.89 that was released. Please proceed as follows:
Once the AVG update is applied reported issue will be fixed.
Based on information from AVG Anti-Virus and Internet Security - FAQ