I don't know how many are left using Mozilla; I also thought this should have been a tip before, but except for Multizilla I didn't find anything. So, Mozilla users, have a look at the mozdev.org
Projects page. There are a whole lot of add-ons for Mozilla; just a few examples:
- Games, like Minesweeper or Solitaire
- Adblock to hide ads (it doesn't prevent them from being downloaded so far.)
- Preferential, a GUI to all of Mozilla's preferences (though some don't work with OS X).
- Leech, a batch downloading utility; just make sure to enter pathways with : as separators...
- Pubmed, a toolbar interface to NLM's Medline.
- Dynamiclinks, create links by control-hovering over a word (try and find out what that means...)
All are easy to install, so give them a try...