Today lifehacker talked about this mac specific application that allows you to run multiple instances of firefox at a whim. By the way their website is at http://davemartorana.com/multifirefox/ I've been running multiple instances of firefox for a while because I have two profiles with different addons, which I use for different things. I have a main firefox profile, a profile that I use most often, and I have an "online gaming" firefox profile which doesn't have as many addons so it's slimmer, and it has some game-specific addons.
For a while I've been looking for a way to run these two profiles separately easily. Initially I copied the code for opening the profile manager (which you just paste in terminal) but what I didn't like about that was the fact that you couldn't choose a "default" profile (I wanted my main profile to be default). Later on I found a website that talked about running an applescript to open a specific firefox profile. That allowed me to set a default profile for firefox but I realized that it took longer than usual to open with the applescript in place so I had to look for another solution. In the end I found multifox (sorry, mac users only but I have a cross-platform alternative that I will talk about later in this post). It's just a standalone application that lets you select the profile you want to run, and what I like about it is that it won't bog your firefox down with an additional addon, as it is an application that runs outside of firefox. This way I can also set a default profile for firefox. I'm going to try using this addon for a while and see how it goes.
And for windows users (or linux for that matter) lifehacker also had another post where they talked about an addon called "profile switcher" where in the file menu it puts a list of the profiles that you have and you can control the behavior when you switch. For example it can close the current profile and launch another one, or you can launch both profiles at once. The lifehacker article is found here. The good thing is that it's pretty simple, and it works with the latest version of firefox. Though what I personally don't like about it is that it makes you install another addon, and in my opinion I have enough addons already.
In the end, here are just a couple of options you can use to manage your firefox profiles or run two or more profiles simultaneously.