Or, it's possible that you don't have an FPT at all and that isn't the problem.
A few months ago, I started up TB one day and suddenly all flames looked like that, unless they had an FPT. It had nothing to do with flame particle textures in individual custom folders, but the default particle for flames was (probably) corrupted. I solved the issue by deleting and reinstalling. If the help above doesn't fix it, try reinstalling TB.