Check with your employer to learn the details of your furlough. In general, here are some guidelines:
You would not be eligible for COBRA if your employer is still offering your group medical benefits while you're furloughed
If your employer decided to discontinue offering group medical benefits to furloughed employees, then COBRA may be an option for furloughed, as well as laid off employees
In other words, if benefits are offered during furlough, COBRA is not an option. However, if your employer does not offer benefits to furloughed employees, COBRA would be an option.