Let's list the most famous ones:
- DownThemAll is still largely irreplaceable since you can download from many parts of the internet much faster if you split the downloaded files in chunks and download them simultaneously;
- NoScript: allows you to whitelist JS execution only for websites that you really trust; JS has been used as an attack and tracking tool since its inception;
- Status-4-Ever and Classic Theme Restorer return Firefox to the time when it was a powerful tool with its own identity and looks, and not a Chrome clone;
- UnMHT add-on allows you to save complete web pages as a single MHT file;
So what will you do less than a year from now?