Normal
You only need to claim mitigating circumstances to law firms if your grades are lower than a 2.1 - you don't need to claim them otherwise.You can reference the circumstances, and in your situation, it will be important to do so anyway in case you need any reasonable adjustments to the recruitment process too.However, you may want to hold off speaking to your professor until you have your grades though, as ultimately, you could be worrying about nothing here - you are not even certain that your contract module will be a low grade.
You only need to claim mitigating circumstances to law firms if your grades are lower than a 2.1 - you don't need to claim them otherwise.
You can reference the circumstances, and in your situation, it will be important to do so anyway in case you need any reasonable adjustments to the recruitment process too.
However, you may want to hold off speaking to your professor until you have your grades though, as ultimately, you could be worrying about nothing here - you are not even certain that your contract module will be a low grade.