Had this problem for a little over a year now (also kept app entirely up-to-date).
This occurs extremely frequently. If a file that is downloaded has the same name as an existing saved file, firefox will overwrite the existing file regardless of folder location.
Depending on how many files have the same name structure (including filetype), it will also randomly “pick” a file to replace from numbered files.
(Downloading “unnamed.jpg” could overwrite any one of the other “unnamed(#).jpg” files you may have saved.)
I often have to paste image urls into a separate browser just to download them without losing ones I have saved.
I’ve seen mentions of this on various forums going back as far as 2019.
It has been acknowledged on bugzilla (bug 1812788)
Still hoping on this getting resolved in a future update.