Rated 4 out of 5 stars
I used the ScreenGrab extension for awhile and then switched to Pearl Crescent Page Saver Basic. Let me say that FireShot is a much more extensive and useful tool than either of those extensions.
The only problem I had with FireShot is that it crashed Firefox (3.0b4) immediately after saving a screen capture. The screen capture was successful, but what a pain! (I see that several other people in the reviews below have also experienced this problem.)
I did some troubleshooting and discovered that the problem is not due to an incompatibility with another extension; it is due to the FireShot button on the toolbar (believe it or not). If I remove the button FireShot works perfectly. If I place it back on the toolbar FireShot starts crashing again. This isn't a major problem for me since FireShot is also available in the mouse context menu.
The aforementioned problem combined with the lack of an undo feature is why I gave FireShot a rating of 9.