It's slightly buggy for Twitter. When I'm in my timeline and apply the bookmark, the age of a tweet gets updated and is shown for about 2 seconds.
I don't have my bookmark bar showing so the arrow pointed nowhere.
I can't drag the button on my Firefox 7 toolbar. Oops.
I made this. FF does weird things with non-text bookmarklets so I used the user agent to show a text-based link as the bookmarklet, only for FF users, but it seems it is still finicky. If you want to see the effect, I'd try Safari or Chrome.