The timing between the books is completely irrelevant. The appearance of the character is what matters here. FF #67 is a 2-panel cameo...nothing more. Yes, a first appearance but in the same vein as ASM #299 as Venom's first appearance. It's a first appearance in a cameo and collectors generally don't value that as much as the first FULL appearance, particularly when the character appears on the cover.
CGC label agrees and it looks like the fanboy market, looking at GPA sales, favors Thor #165 as well.
If we're talking about the first appearance of a cocoon, that's a different story...