Its an addon/extension that changes the version number range of other addons to make them compatible temporarily.
An addon will have a set range of FF versions that it officially works with. This info is a simple text entry in the addons files. You can manually go in and change the range to include new FF versions before its author does. This will make the addon work, though doesn’t account for any real changes the author might include. Its temp. so most useful for smaller updates to FF, say from 3.5 to 3.5.1.
Google for more info and get the tester tool at Mozilla site.
I should clarify that nightly tester tools does the temp change for you, as opposed to doing it manually.