Does it matter if you use first or third person for that chapter? Yes? Why?
You are confusing the reader. That's not a bad thing by itself, but you should have a reason to do it. Just "maybe it's better" is not a reason. Does the switch fit to your story? Yes? How?
It depends on the story, it's your story, so only you can decide. Follow Fox' advice if you can't figure it out. Here are two examples why it could be a good idea:
If you had several flashbacks, using first person for all of them would give the reader a distinction when a flashbacks begins and ends. It would be a special style of your novel and worth considering. But well, it's your only one.
If it is a story teller scene (person tells his tale to an audience, maybe just a few friends), then the first person could work if you want the distance of the story teller POV.
Then it's also possible to interrupt the first-person-chapter with scenes in the present time where e.g. the audience asks questions and the story teller gets into more detail about that.
This does not only interrupt the flashback, it also interrupt the reader. But you are doing this anyway if you switch to first person. It is unlikely to keep the reader full immersed during the first-person-chapter one way or another. The switched narrator's perspective will keep him "outside" the story at a distance ("That's not the real story, just this old guy telling tales.") Again, not a problem by itself, the question is, do you want to keep the reader at bay? Yes? Why?