There isn’t any problem opening the homepage with the latest version of FF. The change is what happens when you open a new tab. No longer can you set those to open your homepage. The only options now , as I have already said, are either to open a blank new tab (option available either via settings of new right side gear icon) or, the new default, one that displays tiled home page links to your most visited and other ‘suggested’ web sites.
If one of those web site home page tiles is blocked by AVAST then any new tab opened will result in the error messages the OP described.
I really think that is the best explanation for the new tab error message with FF. But it doesn’t explain why in the first place AVAST+ FireFox is blocking the web site mentioned whilst AVAST+Pale Moon isn’t. That is the real problem. If that is resolved then it should also fix the error message/new tab issue too.