I don't think any browser comes with macromedia flash player already installed. Its you the user who have to install it to view certain websites. A lot of game companies use flash to make better looking websites & also more interactive for the viewers.
I think you have it mixed up with something like unwanted pop-ups/redirection or really annoying flickering advertisements. A number of programs can be used to make these annoying adds not just flash .
Below is a site made with flash now its not a great example of what you can do but it is good.
example of flash site
Firefox has a built in pop-up blocker that works very well its also a lot more stable than IE6 which is old & made by M$ & uses active x which can be used to do all sorts of nasty stuff to your pc.
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