RSS feed https://rawtherapee.com/feed has a typo that prevent open it in FF.
https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Frawtherapee.com%2Ffeed
Line 20 contains letter “l” outside quotes.
<summary type="xhtml"><div xmlns="https://www.w3.org/1999/xhtm"l>