Being unable to delete is probably due to open library filelocks or assigns. MorphOS cannot delete them because they really are still in use.

That said, why exactly would you want to use IBrowse in 2020? I used to like it in its day, but it's more or less a simple html viewer, not a browser. Lack of CSS is a killer alone, and I'm sure the javascript parser hasn't advanced much from the late 90s.