I don't see the rollback link for this edit.

You should still see an edit link under the question, though -- correct? If you really feel that your original post was better the way it was, you can just edit the question to be the way it was before. That's materially the same as a rollback. If you don't see an edit link for your own question, I really have no idea why, and that sounds like either a possible bug, or some kind of a ban being applied to your account. Since I'm not a diamond moderator I can't provide any further insight, sorry.

Did my question have any grammatical errors?

Yes, it did, but they were fairly minor. The edits being made were rather picky. We've seen much worse than your original question on SuperUser, and while those worse posts are often edited so they're not terrible -- which is a good use of the edit feature -- yours was more borderline. Personally, I probably would not have taken the time to edit your post for grammar; however, I feel that your post is better in some sense now that the edits are complete. Therefore, I don't think a rollback is really warranted.

Weren't the tags relevant or sufficient already?

Well, your question is more about how to view a large XML file, and not about Firefox specifically. All he did to the tags is remove the firefox tag. That doesn't strike me as particularly egregious, since your problem is not primarily with Firefox, but with the concept of, "okay, I have this huge XML file and the vast majority of programs can't deal with it properly....".

Didn't the ones who approved it see that there was no need for revision?

Apparently not. Again, the revisions were minor, but since there were several minor revisions, it wasn't just a single-character edit, so declining it as "too minor" wasn't really warranted.

It's a judgment call. There aren't really black and white rules; we have guidelines which people interpret in different ways. And heck, even if the consensus eventually is that the edit was unwarranted, two random people with edit privileges making the decision to accept is hardly an indication that the system is wrong. People make mistakes. It happens.

I don't know how this can be controlled.

In the general case, we want to encourage edits that significantly improve the content on the site, rather than discouraging them because they violate some notion of authorship or ownership of posts. Unlike a forum, our goal is not to say "I did this!", but rather, to have the best possible content presented immediately to the user when they click on a question link from a search engine. The reputation system, your name attached to the question/answer, etc. are all secondary or tertiary aspects of the site, and serve a much less important purpose than the actual content that is produced (the question and the best answer to that question, most importantly; comments, too, are secondary).

In this specific case, you could make the argument that the edit was not significant enough to be worth anyone's time, but the edit was made, and your post is better off for it. Ending an interrogative statement with a period is a particular pet peeve of mine, and he fixed that, so he gets a cookie in my opinion. OK, so I'm half-joking about the previous sentence. But saying you want the edit reverted is like saying that you intentionally want your question to be worse. The fact that the edit only helps a little bit is not really doing you any material harm. If it were changing the meaning or intent or subject of your question, then I'd be defending your authorship rights.

I'm pretty sure anyone who's either a native English speaker or otherwise could have understood my question the way it was.

I agree. No further comment on that.

I cannot undo the approved edit. I cannot report it to an admin.

The second sentence is incorrect. You can "flag" your own question (by clicking the "flag" link underneath your question text and the tags) to tell a moderator about it. Moderators can always see the entire edit history, as well as who made the edits, who approved them, etc. Based on this, they can decide whether the edit was appropriately approved or not.

You don't have to concern yourself with flagging the edit specifically. Flagging the question itself, and explaining in a few words what you want the moderator to look at, is sufficient.