Hopefully this will also make it possible to have separate instances of the Flash plugin for different sites, rather than a single instance for all sites. In addition to improving security through compartmentalization, having separate copies of Flash will allow Firefox to unload each one when finished with a site. Flash leaks a huge amount of memory, and currently many people continuously have some site open which uses it (often sites like gmail or github that use it for "utility" purposes), which means Firefox can never unload it, resulting in an ever-ballooning memory footprint that people often blame on Firefox.

(I say this as someone who doesn't use Flash personally, but everyone benefits when systems have more security, and I personally would like to see less unfounded complaints about memory usage distracting from the legitimate efforts to improve memory footprint and performance.)