Tab Mix Plus does something to this effect. However there are two issues with it.

First it doesn't seem to be optional so (as I'm writing this) it seems to be coded to simply override the FF4 arrow-under-tab-while-moving-it event. This causes a bug where randomly the default FF4 arrow under the space between tabs will show up, and stay there indefinitely. The only way to make it go away is to restart Firefox. I plan to post a bug report for the Tab Mix Plus devs if one doesn't already exist. I'm unable to reproduce the bug so it seems to happen at complete random.

I attempted to hide the default FF4 arrows manually using a bit of CSS in userChrome.css but I could not find the class name. I already tried searching through the browser XUL files and related stylesheets, and could not find a single reference to this little arrow.

The second issue I can see with this solution is that not everyone would like to install an add-on just to switch back to the old tab-moving arrows. I can guarantee you however that it is a very useful add-on with a large number of neat options that can normally only be achieved by installing many extensions or writing a lot of CSS yourself in userChrome.css. The reason I use it myself is for the ability to scroll through tabs using my mouse wheel.

I've attached a picture of what the tab-moving event looks like with Tab Mix Plus. Hopefully this is the visual effect you are looking for.

Here is a link to Tab Mix Plus: https://addons.mozilla.org/en-US/firefox/addon/1122

sanitysama, the tab drag arrow looks better, but ideally I would like to see it above tabs. This is because during dragging the tab is partially hidden by transparent page pane. I believe that tabs are not drawn on the top, because one can hide Mozilla menu, then there will be almost no space on the top for the arrow ;)