There are many muscles that run around there. Best way to figure out which one it is is to look for distal attachments. FHL runs behind your fibularis under your gastroc and soleus. Try pressing there gently for a tender spot. If you find it it could be FHL. Next is keep pushing on the tender spot and extend your hallucis. If that's the same pain then that's also the way to treat. It's called pin and stretch.
Now if that doesn't reproduce the pain then it's not fhl and you have to search some more. it fh brevis (fhb), adductor hallucis, tibialis anterior, or even fibularis longus. All of those attach to the first ray and are close to the first met head. I use the same tactics. Start where the pain and start tracing muscles away from the site of pain until you again recreate the pain. Hold the tender spot until it's no longer tender or hold it and move your toe either passively or actively.
If none of those work then something else is going on. It's the challenge and joy in physical medicine for me. Figuring out what's going on. Good luck and let me know your results.