Some time in the last few years, this has disappeared. Luckily, with the assistance of extension and add-on developers, this option is available for both Firefox and Chrome!
After installing either browser plugin, an icon will appear if the webpage includes HTML like the following:
<link rel="alternate" type="application/rss+xml" title="Mario Lurig RSS" href="http://mariolurig.com/feed/"/>
You can test it out by visiting this website after installing the extension/add-on.