Me and my brother shares the same PC most of the time and we both have our own profile for firefox. I recently figured out that I can run both profiles at the same time. Profile Switcher is a firfox add on that allows you to separate Firefox profiles and helps you use multiple profiles simultaneously. Here is a great guide by SimpleHelp to help you quickly change from one profile to another and even use both at the same time.

1. Head over to Kaosmos website and install it. As with all Firefox addons, you’ll need to restart Firefox to finish the installation.
2. Select File -> Open Profile Manager.

3. If you’ve never added another profile, there will only be online listed – default. Click the Create Profile… button to add a second one.

4. Review the info on the Introduction screen, and click Continue

5. Give your new profile a name – ideally a descriptive one. Click Done

6. Highlight your new profile, and click Start Firefox

7. Now you’ll have your old (default) profile, and your new profile, both running at the same time. Any addons that you install in one profile, will not be added to the other. The same holds true for bookmarks, browsing history etc.

8. At any point in time if you want to launch a different profile, select File -> Launch another profile and then select the one you want to open.

9. You can also set ProfileSwitcher to automatically close the “open” profile when you launch a new one. To do so, select Tools -> Add-ons and from the Add-ons list, click the Preferences button in the ProfileSwitcher section.

10. Select the behaviour you want Firefox to adhere to when you change profiles (never close the current profile, always close the current profile, or always ask what to do).

11. Now you can simultaneous profiles, or quickly switch between them. Note: for each profile you setup, you’ll probably want to install at least one common addon – ProfileSwitcher. That way each profile you create can quickly switch to another.