I'm running WIn7 Ultimate too and Firefox is my favourite browser also, but Firefox certainly has problems with PeroPero Seduction. It had at least stuttering on animations and crashes during Elite Guard 3, when i've started to play PPS, as far as i remember, so i switched to other browsers for playing PPS. I'd recommend you to use Google Chrome, since it is officially the best browser for Nutaku games as they (Nutaku) say, but i can also advice you to use Maxthon, since it allows to detach Flash elements (like browser games or online videos) from their web pages in the separate, resizeable windows, so you can, for example, detach PPS, shrink it to the size of box of matches and keep that tiny handy window in the corner of your screen all the time, on top of all other windows, to always see your Focus/Stamina status, Elite Guard status, easily hunt Pink Cases during Gambit events or just roll those ~100 N Gacha every day for Daily Flirting Missions while doing other stuff, like chatting on Discord, surfing the Web, playing some othe browser game, etc. (you name it).
Here is an example of how it looks if you do that:

More than that, Maxthon has a built in Resource Sniffer, which you can use to download PPS game files like card images, animations, background music and Movies to your hard drive quite easily (though, most of the resources are packed into .swf archive files, so you'll need additional tools like SWFRIP to extract those files from them) - you can use it to get images for our HBC Wiki like that, for example, or to use card images as avatars here, etc. And of course the Sniffer works not only in PPS... ^_~