There are several reasons to run multiple FireFox windows that are disconnected (not the same profile). Using the same FireFox profile means all windows are connected so all cookies are set the same for each window - which can be annoying at times (for example having multiple accounts with a web site). Another reason could be to separate extensions onto different profiles (in case they conflict or are slowing down FireFox due to the sheer number of extensions you have).
To create an additional profile in FireFox (using Ubuntu) close FireFox. In terminal type firefox -profilemanger
Use profile manager to create new profiles.
Then create an additional FireFox shortcut.
Next edit the properties of that new shortcut: in the launcher tab update the command field:
firefox -P newprofilename -no-remote
where newprofilename is the name you gave your new profile in the profile manager. I hope this is helpful.
I found the following useful in getting this to work in Ubuntu 7.04 and FireFox 2.0: Use Multiple Firefox Profiles at the Same Time - Geek to Live: Manage multiple Firefox profiles
Tags: ubuntu - FireFox - FireFox profiles