Enigmail is an extension to Mozilla Thunderbird and Seamonkey to integrate the OpenPGP standard (specifically GPG) with the MUA.
After enigmail has been configured for a while and you then wish to go back and change a setting it can be quite confusing as to where to do it. The most obvious place to start is the OpenPGP->Preferences menu item, though Edit->Preferences would possibly have been better (I assume this is a technical limitation however) which brings up the following dialogue.
As displayed on the tab label and on the heading just below it, this is the 'Basic' set of preferences, so common or frequently changed preferences should be expected here perhaps. The first preference displayed is to do with overriding the default gnupg program to use, this option is only useful to those people who have multiple copies of gnupg installed and the preferred one isn't found first in $PATH, which seems a little crazy for the first basic preference.
The second preference has slightly more of a right to exist but is also flawed. I'm not sure what 'idle time' referred to here means; time when the computer is inactive, or time when Thunderbird isn't being used. I wonder whether or not it's ever the case that I want the length of time my password is remembered to be the different to the length of time before the computer locks.
The checkbox displayed in this group is something you have to enable if you didn't create a passphrase along with your key, I imagine that this is something that can be figured out automatically when enigmail tries to use the key. Originally I thought that this preference was a way to never remember the passphrase. Possibly the best thing to happen to this preference would be to remove the present widgets and replace them with 3 radio buttons like so:
+---------------------------------------------+ | Remember passphrase: | | ( ) Forever | | ( ) For this login | | (*) Never | | | +---------------------------------------------+
The third preference is a preference to configure the preferences window. this works by adding more tabs on either side of the 'basic' tab at the top of the page when the checkbox is enabled. Some of these settings seem quite unneccessary also, my favourite being 'let me read any encrypted messages I send', I'm unsure what the usecase for 'let me write emails I can then never read again' is.
In keeping with the seemingly arbitary way Thunderbird splits up its own preferences, there are also more preferences available under Edit->Account Settings (pictured below), which generally seem quite sane (though when doesn't one want to sign an encrypted mail?). The most confusing thing about this window is that it has a button labelled 'Advanced...', which when clicked opens up the 'Basic' preferences window outlined above!