Using Firefox 3.0.10, i have many customizations and addons that make my experiences of browsing the internet so much better as i have a great browser to do it with. Here are the first steps to browsing perfection, using addons and customization.
First, visually.
Now i used to use a nice looking Internet Explorer-like theme as it matched my taskbar and general theme. Now, i simply use an extension that makes Firefox see through with an Windows Aero-esque effect. It is here and is a simple click and install job like most extensions in Firefox (which is why i love it). It looks great!

However, i can see this is not for everyone and there are thousands if not hundreds of thousands of themes for Firefox. The main source is of course official and probably has most of them listed. However, you can always fire up your favorite search engine and look for them.
Secondly, bookmarking.
Bookmarking is literally saving a link. However, instead of just copying and pasting them into a big word document, it is easier to have quick 1 click buttons at the top of your browser. Firefox has a built in bookmarks bar at the top under the address bar. I use XMarks to backup mine and have used the Smart Bookmarks addon to make the names disappear cause i have a lot of them.
Thirdly, other extensions that make things easier.
Many people on the internet live with something called Greasemonkey which allows the browser to use various code snippets and enhance browsing. However, i haven't ever tried it. I use other various extensions which i will list here.
The All Glass Addon (Windows Vista and Windows 7 only, Firefox 3.1 beta 3 recommended, works fine with 3.0.10) makes Firefox look Aero-like as detailed earlier.
AnyColor (Most versions of Firefox) allows you to colour or add a photo background to your Firefox.
Google Reader Watcher (Most versions of Firefox) shows number of unread feeds on your Google Reader.
Hyperwords (Most versions of Firefox) makes every word on the internet interactive. Use it to translate, share or a multitude of other things with any words on the internet.
PDF Download (Most versions of Firefox) allows you to use PDF's in many different ways in Firefox. It adds extra functionality to your Firefox regarding PDF's.
Smart Bookmarks Bar (Most versions of Firefox) detailed earlier, makes bookmark names disappear and show on rollover.
Wolfram|Alpha Google (2.0-3.5) shows the new, fantastic Wolfram|Alpha search engine results next to Google Search results instead of those annoying sponsored links on the right hand side.
XMarks (Most versions of Firefox) automatically syncronises and backs-up your bookmarks all the time. Really fantastic. Also gives an icon in the address bar giving you extra information about websites. Can also back up passwords.
Making it all work together
Installing these addons mostly take care of making it all work together.
XMarks will require a user name and password and so will Google Reader.
Add more or delete search engines by clicking the box to the left of the search bar and clicking Manage Search Engines. (Tip: Add Wolfram|Alpha to the search box with this add-on)

Customizing your space at the top can also help. Right click on the top and click Customize and you can have everything you like wherever you like. I have moved my Address bar to the top bar with the File, Edit etc menus, along with my Search Box. I have moved my bookmarks to the bar with the Refresh, Stop and Back/Forward buttons. I also removed my Home button, as i don't use it anyway. It makes for a minimal workspace with more viewing space for your websites.

(You may also want to get rid of your bookmarks bar like I did if you did a similar thing to me by right click > Bookmarks Bar)
Of course many more add ons out there, but this is my lot and how I use them.
Honourable mentions (because i either have a different program/add on with a similar purpose or don't use anymore)
TwitterFox, Google Gears (use the Labs feature in Google apps), Fission, Cooliris, ColourfulTabs, Greasemonkey
Final look:

Any other add-ons you love? Love your Firefox? Give your tips in the comments.