Add the download button and the open button to download or open profile picture and media in the posts, stories, and highlights in Instagram
< Feedback on Instagram Download Button
After a few days with this problem it's just gone... Nothing changed.
Two issues currently (Firefox 120.0.1):
For regular timeline posts (pictures or videos) it doesn't download the opened one but any picture I had on my timeline minutes or hours before.
For multi picture posts the filename for all pictures will be the one of the first picture