The future of Firefox's add-ons arrived in Firefox 4 with the introduction of "restartless add-ons." Based on the new Jetpack API and Add-on SDK, restartless add-ons--also known as bootstrapped add-ons--don't require a restart to be used or removed. Not coincidentally, they also provide Firefox with a venue for competing directly with other browsers, which use add-on frameworks that were created after the technology that supports restartless add-ons was created.
Firefox doesn't offer many restartless add-ons just yet; at the time of writing, there were only 143. Also, there's been documented problems by existing add-on authors in making their add-ons restartless, but that doesn't mean that the ones available are worthless. In fact, some of them add some impressively useful functionality to the browser. Here are 13 of the best.
The entire collection of Prospector add-ons from Mozilla is a great way to take advantage of the flexibility of restartless add-ons. Install one, test it out quickly, and if you don't like it you can remove it as fast as you installed it.
The most interesting Prospector experiment is Home Dash. It's possibly the most annoying, too. Home Dash is a complete re-envisioning of the browser interface, presenting a workflow unlike any other browser out there today that attempts to emphasize tab previews and search queries. It's not for everybody, but it's definitely worth checking out.
The current crop of Prospector Experiments includes Speak Words for location bar search completion; Instant Preview for faster page loading from location bar suggestions; Find Suggest for search suggestions when using the in-page Find tool; and Start Faster, which makes a customization tweak to how Firefox starts when you've just turned on the computer. The short version is that it does what it says it does: Firefox will start faster when you've got it running, at least after you immediately restart your Windows box.
The slightly more technical explanation is that it bypasses the Windows prefetch feature. The add-on will install a Faster Firefox icon on your desktop, which has to be used to start the browser after a Windows reboot. To double-check that it's working properly, the developer recommends using the About:Startup add-on, which is also restartless.
Type "about:startup" into your location bar and you'll get, in milliseconds, how long Firefox's main process takes to start; how long it takes for XRE_main to be called, one of Firefox's first called functions; how long it takes the tabs from the previous session to be restored but not reloaded; and how long the FirstPaint takes, which is the time it takes for the first tab you're looking at to fully load.
Note that about:startup, while technically a restartless add-on, will not be able to display information from before it was installed. In other words, you have to have it installed before Firefox starts to get information from it.
While we're on the subject of restarting, Restartless Restart adds a menu button and keyboard hot-key combination to allow you to restart the browser. It currently lacks a toolbar icon, yet the core function, on-the-fly restarting, is effective and smooth.
LuckyBar restores the "I'm Feeling Lucky" functionality to Firefox 4. The feature was present in Firefox 3, but removed because of Mozilla's concerns over privacy issues relating to accidentally sending search queries to your default search engine instead of restricting the search to your personal Firefox data.
Easy App Tabs helps you create and remove App Tabs, the permanently pinned tabs on the left side of your tab bar. Double-click on a tab to set it as an App Tab, or return it to normal status. The feature was left out of Firefox because Mozilla found that users were accidentally creating app tabs when they didn't want to.
Another smart and simple tab-focused add-on is Tab Badge. This add-on shows you an unread count on your social networking and e-mail sites as long as the number of unread messages is programed to appear in the site's titlebar. So, if your Gmail title is "Priority Inbox - 10," the number "10" will appear in a red circle on the tab. While Gmail has the feature available as a lab experiment you can activate, this is a clever way to get it on all your communication sites.
Developed by Diigo.com, Read Later Fast saves URLs to be read later. It can be synced locally and with the cloud storage at Diigo, and presents an interesting alternative to other "save-for-later" services. However, this feature is built into Firefox 4 in a very basic way. You can use the bookmark star to quickly mark pages, close the tabs, and then use Firefox Sync reopen the starred pages on a different device. That doesn't mean Read Later Fast isn't useful, though.
Long one of my favorite Firefox add-on features has been the capability to drag items from Windows Explorer directly into a text field to upload them. Drag2Up provides that functionality in a restartless add-on, which is great for toggling it off on the rare occasion when the feature interferes with text box functionality.
If you have a favorite restartless add-on not mentioned here, tell me about it in the comments below.