These are my favourite add-ons which I believe aren’t as popular as things like AdBlock, and might be of use to you.

1. The best theme I’ve come across so far is “TwentyTen”

It’s modern, sleek, and uses the Win7/Vista Glass effect quite well.

2. ScrapBook – Saves webpages to your computer with a click.  It’s like bookmarking, but it actually keeps a record of the page.  It’s useful because you don’t have to worry about the page vanishing between visits.  I mostly use it to keep track of recipes, hah.

3. SpeedDial – Creates thumbnail links of your most important/frequently used webpages in new tabs (like Opera).