I get asked versions of this question a lot.
Here’s the thing of it.
At this point, depending on how you choose to look at it, the Marvel Universe is either 53 years old or 75 years old. Either way, it’s the longest running shared fictional environment every created. (The DC line didn’t really become a “universe” per se until really the early 1980s or so.) That means that more people have been contributing to it more consistently than anything else in existence. And the larger that tapestry grows, the more impossible it becomes for absolutely everything to exist within a perfect non-changing absolutely static “canon.”
Past a certain point, the question of “canon” really becomes: will future creators reference it? And the answer in most cases is likely “somewhat.”
Inevitably and inexorably, there’s a magnetic pull to the original stories. Those are the ones that more often than not exert the strongest pull on the history of the character—those are the ones that creators and editors go back to whenever devising any retellings or history implants or anything along those lines.
But often, there’s some degree of “seepage” from the newer retellings, depending on how well they’re done, how well they’re received, and how well they address the needs of the audience and the creators of right now. So for as many times as we’ve told it and expanded on it, the core origin of the Fantastic Four is what we saw in FANTASTIC FOUR #1. However, the core origin of Daredevil incorporates all of the additions that Frank Miller made during his assorted runs on DAREDEVIL.
So it varies.
In the case of THOR: FIRST THUNDER, its storytelling mandate was very similar to the later THOR: SEASON ONE book. Both of them recount the earliest days of Thor’s time on Earth and his earliest adventures, weaving among the original stories from JOURNEY INTO MYSTERY. And yet, the two don’t really match up very well.
So which one is “canon”?
The answer is both, to some degree. And neither, to some degree.
it really all depends on what future Thor writers choose to pick up on from either of those retellings and use again in new modern stories. Inevitably, there’ll be cases where Writer A will reference one, and then Writer B will reference the other—depending on which story they preferred or which project they read at a formative period in their lives.
So really, this stuff functions these days more in the manner of legends. If you compare all of the assorted Norse myths, there are a bunch of outright contradictions. And yet, all of them are “true” so far as Thor and Odin and Loki and everybody are concerned.
That’s how the Marvel Universe operates at this point as well.
So, final answer is yes. But it’s also yes for the stories that contradict it.