This addon is no longer maintained. Version 0.6.0 is available from the versions page, but it was never fully approved by AMO, I don't remember why. Any version of this addon may or may not show accurate information in the latest FF/TB versions. Sorry, everyone.
This addon does exactly one thing: displays Firefox or Thunderbird compatibility versions for each addon in the addons window (Tools Menu -> Add-ons).
It also adds the ability to copy an extension's ID to the clipboard ("Copy ID" in extension's context menu). If you only need to quickly reference the ID, it's available as a tooltip on the compatibility range.
Works for extensions, themes, and locales.
Google group: http://groups.google.com/group/isitcompatible
Source code: https://bitbucket.org/eternicode/isitcompatible/