firefox.exe -P

I don't know why your profile is missing. If you recently did system updates, please shut down and restart Windows in case there is a permission problem blocking Firefox from accessing your profile. If that doesn't help, you can at least start browsing by creating a new profile. '''Create a new Firefox profile''' A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should have completely fresh settings databases and a fresh cache folder. Start up in the Profile Manager using the Windows 7 Start menu search box. Type or paste the following into that box and press Enter to execute it: firefox.exe -P ''Don't delete anything here!'' Any time you want to switch profiles, exit Firefox and return to this dialog. Click the Create Profile button, assign a name like October2017, and skip the option to relocate the profile folder. After creating the profile, select it and start Firefox in that profile. That should at least get Firefox running. Assuming it keeps working after you exit Firefox and start it up again, as the next step, we can suggest there to look for your old data.

Thanks!