Like many 'builders', I have experienced and heard of numerous issues using the FIRESTORM viewer. after uploading Windows 10.
'settings' revert back to default.
extremely slow frame rates and response times.
Unable to log-in.
Having spoken to Firestorm Support, they are aware of the situation and are working on the issues and will be releasing a Windows 10 Viewer download in the near future. In the interim, they are recommending a 'clean re-install' of their viewer which seems to solve a lot of the issues.
There is a huge community within Second Life who are 'homebound' disabled and Windows 10 upgrade has caused them all kinds of anxiety resulting in them reverting back to 7/8.1