Upgrades the findbar to allow searching in regular expression and foreign words with the keyboard. For example: You want to find sól, you only have to type sol, or if you want to find たいおtype taio. This addon also highlights the words on the scroll bar. Also see Find In Numbers 
This addon will notify you with a message (akin to the Downloads complete message) if any page on its list has been changed or updated since your last visit. This is very useful for monitoring pages like blogs, your Wikipedia watchlist, special pages (like user contribs, logs), and others. If you keep Firefox minimized, it will still notify you while you are typing a report, watching a video, or otherwise not using Firefox.
There is a defferent version, Update Scanner, which has a nicer GUI but has slight issue when monitoring the watchlist since it notifies you when text is also removed, so when your watchlist removes entries, you will be falsely notified. Update Scanner: 
This allows you to configure tabs any which way you can ever think of. A useful function is that it backs up all your tabs so if you close one, it can be retrieved later. Other useful features include cloning tabs and load bars on tabs. If you have this, you can get rid of most all your other tab related addons.
This addon will notify you with whenever there's a new message in your inbox (akin to the Downloads complete message). This is handy because I no longer have to have Yahoo! as my homepage and constantly check my inbox. There's also one for Gmail but you'll have to search for it.
Enlarges the screen with nice toolbar buttons. Firefox has a built in zoom but it only increases the text size. This will increase the size of images (and svg), tables, movies, etc all proportional to the text.
This is such a useful addon for web developers, it's a must have. So many tools will be open to you once you have this. But, don't load this if you don't web develop since it slows flash down a little.
You know how you can customize Wikipedia through your monobook.css? This lets you customize anything through "firefox.css". The browser, individual pages, etc. There are also wealthy amount of presets. You have to try this to know what it is! This is beautiful and this is useful.
Trims history pages and truncates them. Color codes your entries. Install in .js
Puts date and time in UTC on the top right corner. Better than the gadget version in my opinion.
importScript('Wikipedia:WikiProject User scripts/Scripts/Time');
User contribs apps
Implements very useful tabs when viewing user contibs such as a link that counts edits, shows what hour the user typically edits (example), and, the most useful feature in all the codes for Wikipedia, show/hide top contrib, which shows what pages the user is the most recent editor to, useful on your own contribs
Highlights the names of administrators. Useful for knowing who to respect ;) The highlight color can be changed at .css (example)
This will convert any image (even photos) into a vector version automatically after you guide it through some steps. This website provides 2 free conversions per email address...time to make more email accounts