If your firefox profile has become corrupt, inaccessible or is affecting Firefox’s performance then you may want to create a new profile. When you create a new profile, it will setup Firefox (like new). However, your data, saved passwords, bookmarks and everything else including extensions and plugins will be deleted. You will need to re-install them on the new profile. You can use MozBackup to back up your existing profile (later).
Before proceeding, make sure that Firefox is not running and is closed. This method will work on Windows 7/XP/Vista & 8
Launch the Firefox Profile Manager
a) Hold the Windows Key and Press R to open Run Dialog. In the run dialog, type firefox -p and click OK.
b) Once the Firefox Profile Manager launches, you will see previous profiles listed if available but you do not need to select them. Click Create Profile, then click Next and Finish.
This will create a new profile, which will appear at the bottom of the list (if there are previous profiles, if none then this will be the only profile).
Select the new profile, and click Start Firefox.
You can also visit the YouTube Video For This Guide @ Your Firefox Profile Cannot Be Loaded
If you are a Thunderbird user, take a look at Backup Guide for Thunderbird