First a little disclaimer: install any addon at your own risk, and if your browser slows down, freezes or crashes, try disabling your addons one by one and see if that helps! :D
Not that I personally have any problems with any of these addons (as of this writing) but hey, everyone's system configuration is different! ;)
On to the addons.
If you have accounts at thousands of websites and a not-so-brilliant memory like me ;) then Roboform is what you need! But that is just one of its aspects. One great feature of Roboform is that you can use it to save and fill forms.
To give you an idea:
If you use the Rapid Action Profits script, you know that as much brilliant as it maybe, it doesn’t support "product cloning feature" like DLGuard. So, I use Roboform to clone RAP products. What I do is just save the settings of an existing product into Roboform, and then use those settings to create a new product, with some minor manual edits here and there of course! ;)
BTW, you don’t need to download and install any addon separately. Once you install Roboform it automatically installs the necessary addons for IE and Firefox.
Only downside is that it does not support flash forms as of now. Flash is becoming a rage now and I hope the Roboform guys realize that soon, or I may have to buy yet another piece of software :(
2. Download helper: If you are a Youtube addict like me, then this addon is a must have! There are so many "flash-catcher" addons available for Firefox but this is what I have found to be the best. While download speed is not that great, it manages to grab flash files from several popular websites with little trouble!
One thing to note is that you would need to add the file extension manually when saving the file (such as .flv for flash files), as this addon does not currently have the ability to detect file extensions automatically!
3. Google Toolbar for Firefox: While there are a lot of stuff you can do with this toolbar, I primarily use it for….you guessed it, searching through the forest called "World Wide Web"! :D
If you use its "Autofill" function to fill forms, DO NOT use it to fill up sensitive data such as credit card data! IMO it is simply not secure enough to protect sensitive information like that; for such things I would recommend Roboform.
4. Orbit Downloader Addon For Firefox: There are many download accelerator extensions for Firefox available out there, some of which hardly accelerate your download speed, while others, even though helping you with the "acceleration" part, freezes Firefox frequently. If you want a true free download manager with little headache then Orbit is the one to go for. It does not support P2P networks fully, as far as I know.
BTW, you don’t need to install any addon separately. Once you download and install Orbit, it automatically install the necessary addon for your browser!
Free Download Manager is yet another free tool you can try out (note that some antivirus softwares do flag this software as a "Trojan" but this is just a false positive)!
5. Amazon S3Fox Organizer Plugin: Imagine being able to access your Amazon S3 account through a browser tab. Well this addon does that and more! Its clean familiar FTP-like interface is what I like most! ;)
6. Search Status Addon: SEO toolbar for Firefox I have been using ever since I switched to Firefox. I mainly use to find a website's pagerank, nofollow links, etc.
8. Autocopy for Firefox: If you have been a member of Angela's backlinks membership or Paul's Quality Backlinks membership for some time, you know that some of those "backlink websites" offer a WYSWYG editor to post comments, articles, etc. If you use Firefox then you must have noticed errors when copying and pasting texts into these external WYSWYG editors. I know I did! FF seems to have a problem in copying and pasting text in and out of external WYSWYG editors. That is when I began searching for a possible solution and found this as addon as a boon!
This addon is also helpful for those who are too lazy to right-click every time they need to "copy" text. With Autocopy, all you need to do is to select anything on a webpage and it's automatically on the clipboard! No more ctrl-c, no more right click->Copy, no more Edit->Copy.
To paste text from the clipboard, all you have to do is to right-click into a textbox or other text input area. It even supports "multiple clipboards" feature, meaning that you can have more than one "clip" in your clipboard (by default Windows™ can store only one "clip" at a time in its clipboard)!
9. Archview addon for Firefox: Ever wished you could sneak-peek into a large .zip file before downloading it? OR, have you ever wished you could test out your .zip downloads to make sure they are working okay? This addon is perfect for testing zip files, although it does not always work with .RAR archives. Also, I could not make it work to test a .zip file located within the parent .zip file.
Side note: I enable this addon only when I need to test a .zip file; if I keep it enabled forever it proves to be a nuisance ;)
10. LeechBlock: Do you spend more time visiting unproductive sites than making money? Get rid of these 'leeches" permanently with this addon!
11. AdBlock Plus: Hate those pesky popups and banner ads? Perhaps those Adsense™ ads tick you off big time? Well then this addon is for you!
12. FlashBlock: Same as AdBlock Plus, but for blocking Flash ads!
You can find more top recommended Firefox addons here.
If you find any of these addons useful feel free to post a nice comment here :)