I use firefox 3.5, but the answer may still be relevant.
I use the following combination:
- NoScript addon
- AdBlockerPlus addon
- ImgLikeOpera addon
- Ghostery addon
- Squid web proxy
Between them, they made a big dent in my internet data usage.
NoScript is handy because many scripts download more scripts and images. It also blocks flash. It shows you when it has blocked anything and it lets you unblock them.
AdBlockerPlus stops the browser from downloading ads and the associated images.
ImgLikeOpera gives better control over which images are loaded that the built-in "don't load images" setting. For example, you can tell it to show images that it has already downloaded. This means you can load the images for buttons and the like, and then switch back to "show cached images" mode, and the images are still visible - even when you open the same page later on.
I also use the Ghostery addon. I have it installed for privacy, but I think this stops some (small) downloads.
I have also installed a web proxy (squid). However, it is not that effective: only about 10% of requests hit the cache, the rest are misses that must be fetched from the internet. Still, it helps a bit. The biggest benefit from using the web proxy was analysing its logs, which led to changes in my browsing behaviour (e.g. switching to "basic HTML" mode on google mail, and not leaving a google mail window open), but going through the logs takes effort; it isn't trivial.
Ghostery's lists grow all of the time, so I let it autoupdate its list so that it remains effective. I decided to leave "automatic add on updating" turned on, so that the AdBlockerPlus is updated. I think that on balance this uses less bandwidth than if I left auto-update off, and I don't have to remember to update them myself. However, I could be wrong about this one :)
This eliminates most of the bling, but it does reduce functionality. Some sites only work if I temporarily enable the trackers or ads. Similarly, some sites are almost unusable without images, but some of them become bearably usable if I select "No page style" on the view menu. I decided to live with this level of inconvenience.