In no particular order, here are eight other of our favorites:
Weave Sync is another project from Mozilla Labs, although it's not as clear whether it will eventually end up in Firefox as a default feature. This homegrown tool for synchronizing Firefox across computers and devices introduces incremental syncing and a more-streamlined, less-obtrusive experience, fitting in smoothly in your Options pane. Although it still conflicts with some extensions, including the massively popular and arguably more essential AdBlock Plus, in general it works well and brings a long-missing feature to Firefox.
Multi Links is simple in purpose, but so effective that it's one of the best add-ons of the year. Right-click in a browser tab and drag it, highlighting multiple links in the box. By default, selected links open up in new browser tabs, although you can go into the options to choose whether you want them to open up in new windows, or be bookmarked instead. You're also able to change the color scheme of the box, and the outlines of the selected links, just in case you're into that sort of thing.
Originally known as SmarterFox, FastestFox is a multitasking fiend that helps make searching, pasting, surfing, and downloading faster. Highlight a word or phrase on the Web page and FastestFox will display a bubble filled with search engine icons. After a few seconds of inactivity, the search bubble fades away. The add-on automatically merges linked pages into one, which some users prefer for reading long articles, and it also allows you to check other search engines from any single engine's results page.
Whether you're looking for an unobtrusive panic button, or your just need to clear your screen of those 153 tabs for moment, HideTab can help you out. You can hide all of them at once, or merely one--just don't forget that the hidden tabs are still running in the background.
WebReview makes your start page smarter and more suggestive based on past browsing habits. It's a bit like the Speed Dial feature in Opera, Chrome's new tab page, or Top Sites in Safari, but Firefoxified. It tells you the last batch of tabs you had open, along with most visited pages. But it also shows you a group of sites you visit daily, along with a suggestion of sites you may be interested in going to. It sorts these out by what day it is, along with the time.
WebReview also offers a replacement history tracker, allowing you to search by domain or number of visits. Sites in the WebReview history come with thumbnail previews. Lastly, there's a Graph View, showing the breadcrumb trail of how you went from site to site for that entire session. You can also go back to specific days and see a large graph for the entire day. It's visually appealing and exploratory at the same time.
Wikipedia Diver hooks deep into your Wikipedia browsing to provide a fascinating look at what you've been researching. It organizes your Wiki searches down to the day, order, and session in which you visited the sites, making it easy to revisit old entries. Fortunately, all this data is kept on your local computer and not in the cloud, so there are no privacy issues. The reasonable offshoot of that is that it doesn't track external links you click on from within a Wikipedia article, but that's a small price to pay.
I use URL Tooltip in conjunction with several other, not-new-in-'09 add-ons to maximize my screen real estate when browsing. URL Tooltip is new this year, and is quite savvy for those with larger monitors. It reveals a link's full URL as a mouse-over tool tip, thus allowing you to hide your status bar at the bottom of Firefox if you've got nothing else in it. Along with Personal Menu and the Stop-or-Reload Button, and removing the search bar, I've been able to see more of what I want to be looking at on my screen when browsing.
Have a suggestion for the best new Firefox add-on of 2009? Or think I just got it all wrong? Tell me about it in the comments below.