From my testing there needs to be a piece of code added to the web site for Firefox to find the RSS feed MR has. I did some testing and this piece of code seems to work. I inserted it right after the </TITLE> tag. LMK if this works!
<LINK REL="alternate" TITLE="Macrumors RSS" HREF="https://www.macrumors.com/macrumors.xml" TYPE="application/rss+xml">
<LINK REL="alternate" TITLE="Macrumors RSS" HREF="https://www.macrumors.com/macrumors.xml" TYPE="application/rss+xml">