Still no idea what Sparkle is.
Sparkle is the apparatus developers use to notify users of updates and install them.
When you see
this screen, Sparkle is at work.
All that is is the sparkle-project.org's homepage.
Still not sure how that might affect or be of interest to me in terms of guarding against malware and the like.
That home page answered your initial question, and this, I think, is the answer to your new question...
Any time one of your apps pops up a Sparkle dialog box you
may be vulnerable to a MItM attack
if you click on "Install Update".
According to
alternaut's linked doc, you
may be safe if you've launched Firefox at least once in each account in which you use Sparkle, but as far as I can see, the best approach to dealing with the vulnerability (which, by the way, is the result of a flaw in OS X, not Sparkle) is to simply avoid using it unless you're
100% certain that the app asking to be updated is asking via a secure version.
Use MacUpdate or the dev's website instead.
And if an app asks to be updated to a secure version of Sparkle via a vulnerable version...
