When it comes to Firefox’s plugins, everyone have their own preferences. For me, these are the ones that I like and use the most and according to me are useful for all web-developers are:
1. Adblock Plus – Removes all kinds of ads from web pages. Saves bandwidth and page load times.
2. Add n edit cookies – Manage your cookies with this.
3. Amazing Media Browser – Find out all the embedded stuff on webpages including flv videos, swf animations, music etc. and download it with your favorite download manager. Flashgot plugin makes a great combo with this.
4. CookieSwap – To manage multiple accounts. Switch your account with just one click.
5. FEBE – Backup all your plugins, config, bookmarks, settings etc. with this plugin.
6. Flashblock – Partially solves Firefox memory leakage problems and also saves precious bandwidth by blocking flash. Replaces them with a button which when clicked loads the flash.
7. Flashgot – Integrates most of the download manager. Excellent plugin.
8. Greasemonkey – Use scripts on webpages to change appearance/functionalities etc.
9. Live HTTP Headers – View Request and Response Headers being sent/received.
10. User Agent Switcher – Change User Agent.
11. Web Developer – Extremely useful plugin for a web developers.
Feel free to add to the list!