I use the Adblock edge plugin on Firefox.
Opera does a fairly good job of blocking ads by itself.
With really dodgy websites I enable the Noscript plugin (both on Firefox and Opera) where you can continue to use sites you trust by whitelisting them very simply.
Also use a switchable Flashblocker. I use Preferences Toolbar (Prefbar) for Firefox because I have direct control of many other functions for safer browsing.
Opera can be set to enable a Flashobject only after you click on it.
...but if you can live without Flash then delete the plugin and all the better.
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