I cannot subscribe to any pages
No matter what page I go to, the "subscribe to this page" button does not appear. Also, I put a "subscribe" button in my options toolbar, but it stays light gray (unclickable) for all pages. Have I somehow disabled this feature with some other conflicting option?
Hmm, your Firefox identified itself to the forum as version 27. An update is available: this article describes how to get Firefox 48.0 using the "About Firefox" dialog on the help menu: Update Firefox to the latest release.
If something is holding you back from upgrading to Firefox 48, please let us know so we can suggest solutions or workarounds. Version 27 is not secure; Mozilla discloses security flaws after each new release.
Sometimes Firefox reports the wrong version because that information was saved in a preferences file. If the "About Firefox" dialog (see first article link) shows Firefox 48.0.2, you may need to clear out that incorrect information. See: How to reset the default user agent on Firefox.
And to be honest, I don't remember how RSS worked in Firefox 27!