Nightly Tester Tools has one killer feature: It allows you to resurrect dead Firefox extensions. If you've upgraded your browser from an older version, and the latest build has killed a much-needed add-on, this plug-in lets you return them to life on a case-by-case basis.
Nightly Tester Tools does other things, as well. It gives users the ability to control the titlebar display, takes screenshots, can analyze the leak log, copy the build ID to the clipboard, and copy the list of installed extensions to the clipboard. These function well enough, although we wouldn't recommend the plug-in for them without the compatibility override option. They can be hidden in a submenu under Tools in the Menubar.
Nightly Tester Tools works on Thunderbird and Sunbird, too. Of course, not all extensions can be brought back to life, but when the problem is only a small tweak such as compatibility setting, saving the day is as easy as installing a new plug-in.