It is weird - the press release itself notes that this is the first such acquisition Mozilla has ever made.

In their defense, it won't be the first time they've gotten into operating a service -- there's Sync, for instance. So if they see expanding FF's bookmarking abilities as a key strategic differentiator, buying an existing read-it-later service they're already familiar with could be a simpler/cheaper way of getting there than building one from scratch.