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 – ???