To sync your add-ons, you now can use the same tool that syncs bookmarks and other data. Once you're signed in to your Firefox account, you click the Firefox button at the upper left of the screen and select Sync Now on a Windows computer or choose Tools --> Sync Now on a Mac, and the browsers will sync your add-ons as well as bookmarks and other data. That means you will not only sync any data associated with the add-ons, but install the add-ons onto browsers that don't currently have them installed.
I found that the new tool generally worked as promised. When I began, I had Read It Later and Evernote add-ons installed on my Mac but not on my Windows PC; after syncing, they were automatically installed on my PC. Similarly, I had the Email This! add-on installed on my PC but not on my Mac, and after syncing, it was automatically installed on the Mac.
Not all of my add-ons synced between my Windows and Mac versions of Firefox, however. Interestingly enough, I couldn't sync several security add-ons. For example, the HTTPS Everywhere security add-on that I had on my PC was not installed to my Mac. I had similar problems with a couple of McAfee add-ons, including SiteAdviser and ScriptScan (which admittedly has had some issues recently).
Sync falls short in another way. There's no indication that the sync is occurring, so the only way to know that it worked is to check the browsers on your different computers and devices. Some indication that it is actively syncing, including progress, time to completion and the results, would be very useful.
You can customize what syncs on your browser via the Sync Options screen. You get to it on a Windows PC by clicking the Firefox button on the upper left and choosing Options --> Sync. On the Mac, you select Firefox --> Preferences --> Sync.
There are a number of other improvements. For example, Firefox 11 lets you import settings, cookies, history and bookmarks from Chrome to Firefox. (Before this version of Firefox, you could import all that from Internet Explorer, Opera and Safari, but not Chrome.)