Now FF 57 (Quantum) has been released, old style extensions are no longer supported.

Here are replacements that I’ve found:

• Feed RSS mini icon on Bookmarks Button – Awesome RSS
• DownThemAll – Download Star
• Click-to-Play Manager – ???
• Copy Plain Text 2 – Copy PlainText
• FireGestures – Foxy Gestures
• PrefBar – ??? Might require multiple addons
• ProxySwitcher – Proxy Switcher and Manager (annoyingly doesn’t support PAC files)
• Stylish – Stylus
• Pocket – In My Pocket
• wheelclear – ???