Thank you for that confirmation. I believe the emails were deleted accidentally but since you also removed them from Trash, they could be gone forever.
Your last and only option here is to submit the recovery form that Google created in cases such as yours. This is only intended for emails missing due to being hacked but this will also work for accidentally deleted emails provided that the missing emails are still recoverable from the Gmail server.
Here is the link for the Google form to submit:
Also, it will be worth trying to locate the missing emails using the web browser like Firefox that you mentioned. Please follow the procedures on this link:
In the page above, please try the section under "Common Fixes" to see if your emails are still there but were just filtered or moved to Spam.
I hope that helped.
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