This is easy. Just follow these steps:
First, click on your existing firefox desktop icon and create a second desktop icon from it and name it "Profile Manager."
Then, open that icon and click on "properties>Shortcut tab>"Target" field. In the target field, make the path read like this: "C:\Program Files\firefox\firefox.exe" -p (So long, of course, that you have saved it to your C drive. Really, just add the -p to the end of whatever is in the target box.)
It's the p switch that brings up the profile manager. Now click Apply>OK>launch Firefox>follow Profile Manager prompts.
Now, this is very important: Make sure that firefox is not running in the task manager, sometimes, the first time, it does not shut down completely. (which is part of the problem that forced you to create a new profile.) What I had to do was to close it using the task manager to make sure it was completely closed, the very first time I tried this.
Anytime you click on that desktop icon, with all FF browsers closed, you will get a profile manager dialog box. You should get a box showing the existing profiles and some options to modify them.
Using this profile manager should allow you to get back to your default browser.
I hope this helps.