Firefox Extensions: Increased Functionality

I use Xmarks to sync my bookmarks to an online server, in case I'm ever on someone else's computer and I can't remember the name of what I was looking for.  If you're completely unconcerned about security, it will also encrypt and save all of your passwords, but I don't want that information out on the web.

If you ever want to get temporary access to a website, use BugMeNot.  It has usernames and passwords to many sites, like the New York Times.  There's no need to create a free account just to read an article on a website, so use this extension.

Here's a developer who's made several helpful extensions.  Most notably the "Better ..." series adds tons of minor tweaks to each of the websites.  For instance, GCal has a feature to collapse the header and sidebar, allowing the calendar itself to use the entirety of the window.

And best of all is Greasemonkey.  Greasemonkey is a portal into a whole world of tweaks for websites.  In fact, most of the tweaks in the "Better ..." extensions are really just a compilation of the best userscripts for Greasemonkey.  You have to have Greasemonkey installed, then you can go to userscripts.org and start downloading stuff to make the internet wonderful.
Here is one script that prevents Youtube videos from autoplaying, but still allows them to buffer.  It also removes ads, and automatically selects the highest quality with which to play the video.
And after you install this userscript for Facebook, you will want to give me a big giant hug, and also give me money.  I'll let you play with all it's awesome options, but most notably, it will show any picture on facebook at full resolution just by hovering the mouse over it.  I repeat again that you will want to kiss me once you see how great that is.