A couple months ago, Apple started migrating the App Store to the Web, meaning you could read all about an app without having to actually load iTunes.
Why, then, does clicking an app link open the corresponding iTunes Preview Web page--and then load iTunes?! That's what I call a hassle of the highest order.
Fortunately, it's possible to stop this annoying behavior, to read an app's Web page and then run iTunes only if you want to. The follow instructions are for Firefox; Chrome and Safari users should check out the how-to guide on The Apple Blog. (Sorry, Internet Explorer users--haven't found a solution for you yet.)
1. In Firefox, open a blank tab, type about:config in the address bar, and then hit Enter. Accept the warning that appears.
2. In the main preferences area, right-click anywhere and choose New, Boolean. Type the following name into the blank field: network.protocol-handler.warn-external.itms.
3. Click OK, then choose the true setting in the box that appears.
4. Restart Firefox and you're good to go.
The next time you click a link that leads to an app's Web page, you should also see a dialog box asking if you want to launch iTunes. Click Cancel and go about your business (i.e. viewing the Web page).
If you do decide you want to see the app in iTunes proper, just click the blue View In iTunes button on the left side of the page.