How to temporarily block a web site for testing lost connection

At my work, I need to test whether a web application return proper errors when the connection is lost.

I searched and tried several site blocker add-on for Chrome and Firefox, however, most of them are design for blocking the site that aren’t work related, but I need to find the one for work related web sites. 😛 In other words, I am looking for the add-on that provide an easy way to toggle the block/unblock function, so I can work.

I have tried the following:

1. StayFocused (Chrome): This one let you set time limit on the web sites that you try not to spend lots of your time on. However, it does not provides the easy toggle.
2. Temporary Site Blocker (Chrome): This one is pretty close what I need, it has a button and a clean menu to let you enable and disable the blocking. However, it does not block the action on current page, i.e. you can pretty much to anything before you leave the page. Not I want.
3. BlockSite (Firefox): This one does not provide a cute icon in the UI that do the toggle, however, I can check/uncheck the “Enable BlockSite” in other tab to achieve the same.

So far I am using BlockSite for this purpose. But I guess there should be some better add-on for this.