Firefox 2 has a lot of small usability changes, that are great. I’m sure we’ve all accidentally middle-clicked on a bookmark folder, or live bookmark, only to be bombarded with many many tabs opening many many web sites at once. In Firefox 2, if you trigger an action that will open 15 or more tabs at once, Firefox will ask for confirmation.
But what if you want to change that threshold? Well there’s a preference setting for that.
Enter about:config in the address bar. In the resulting page, search for the preference browser.tabs.maxOpenBeforeWarn.
Double-click on it, and set the value to the number of tabs in which you want to start receiving the warning.